BLANK FLANK: The state of not having found your calling (and thus your cutie mark). Used as a mild insult, implying the target hasn't grown up yet, or has no direction in life.
BOARSERKER: Shetland version of a berserker. Terrifying to go up against and fight alongside.
CIDER, DAY AND NIGHT: Ponies classify brewed beverages into two types: low or no alcohol ones are termed 'day' and those with significant alcoholic content are termed 'night' – since ponykind frowns on imbibing such when there's work to do. Despite the existence of the Ponyville Dam, reticulated water is still relatively new in the area, so some ponies still accompany their breakfasts with a day cider or a day beer.
CLAN: A Shetland herd bounded by geographical region, comprising several families and ruled by a single leader known as a Thane.
EQUESTRIAN: A member of the Equestrian race; a pony of Equestria. Technically you could argue that cattle and bison are also Equestrians, but normally it refers solely to the pony species.
EQUUS AND EPONA: The original god and goddess figures of the Equestrian 'old religion'. These beings formed themselves, then everything else, from the primal Chaos, as well as waging battles against Them.
Equestrian religious systems are very much a blank slate in canon, but it makes sense to assume that beings comprehensible to us will share impulses we can recognise, such as the spiritual one. This despite effectively being ruled by two 'living goddesses'.
There is a small cult that considers the Royal Princesses to be the descendants of Equus and Epona, but it is not prevalent or taken seriously.
FLANK: Hindquarters. Equestrian equivalent of buttocks.
FLUFFY: A highly offensive term to anypony, casting aspersions on their intelligence, breeding, and usefulness to society. May also be an allegation of vanity. (Based on the entry at wiki/Fluffy_Ponies)
FOURTEEN O'CLOCK: Noon. Ponies in this fiction use an octal counting system (four hooves, two ears, one nose and one tail). As such 10am pony time is 8am human, 14pm is our 12pm, and so forth.
Pony clocks in reality have thirty divisions for hours, traditionally with midnight at the bottom of the dial and noon at the top. Assuming a similar rotational period, the Equestrian hour has forty-eight minutes of 75 earth seconds' duration.
GELD: Offensive term slandering a stallion's virility. The female form is 'barren'.
GOBSHITE: How a Shetlander tells you that you would be best advised to reflect more carefully on your statements before sharing them; also, that continuing to air such sentiments may result in you being hung by the tail from a tree with hoofmarks all over your head.
GREAT ONE, THE: An extremely dangerous (and large) member of Them From Outside imprisoned under the Shetlands. According to hearsay, the entire array of standing stones, dolmens, and circles comprise a magical net or cage to keep this being transfixed. For reasons best known to themselves, the Royal Princesses keep the known facts about this and possibly other beings secret.
HAME, AWAY: Shetlander expression of incredulity or cynicism. Equivalent to 'Get out of here!'
HARMONY INCARNATE: The sextet of ponies who embody the Elements of Harmony. More of a formal term, but still used interchangeably with Elements references. As such Twilight is Magic Incarnate, Fluttershy is Kindness Incarnate, and so forth.
HERD: General Equestrian term for any grouping of ponies. More specific terms do exist for particular types of herds.
HOOF: The baseline unit of measurement, about four inches. These are grouped into units of four, called a 'stride' or 'fourhoof' (as opposed to a 'threehoof', or one human foot). As such, ponies tend to use a "hoofstick" of four strides, or in human measurements 5'4" (or 1.626m). A typical adult Equestrian, judging by fanmade imagery, stands between three and four hooves at the shoulder.
Given that ponies have no digits and are quadrupedal, a base four numbering system makes sense; however if they included ears, nose and tail, it could be base eight. (In this fiction, ponies use an octal counting system. See FOURTEEN O'CLOCK.)
It is possible that pegasi can count in base ten, and unicorns in base nine, but that is not explored here.
HOOFBALL: A popular team sport resembling rugby league with some influences from netball, and a sink for aggression. Being quafrupeds, the ball is carried in the mouth, but the rules on scrums, tackles and so forth are reasonably similar. Pegasi play a similar game called Cloudball with rules taking flight into account. Magic is expressly forbidden, and pegasus and unicorn members are required to wear 'binders' to negate any advantages their species might give them. Varieties of 'Hornball' do exist, but so far none of them have reached anything beyond niche or experimental status.
The main source of difference is that the rules are designed for quadrupeds. As well as mouth carry, kicking games tend to be races against time before the referee calls a dead ball (or it gets turned over).
Hoofball players tend to have very strong neck and shoulder muscles from all the passing they do, since attempting to pick up the ball, or kick it with a forehoof, can result in either penalty or temporary detention beneath the opposition.
The field is slightly longer than a human rugby field, being 1746 hooves (octal) between goalposts and 1262 hooves (also octal) wide.
INHABITANT OF THE MOON: Equestria's primary defence against Them From Outside. Not particularly intelligent, Luna's main duty is to prevent the Inhabitant from rotating its prison so that it can browse the surface of Equestria.
LAIRD: The highest position of Shetland society, this pony effectively leads the entire Shetlands as a herd, and has the absolute power of veto over decisions made at the Council of Thanes. Typically a Laird once chosen rules either until death, or stepping down due to disgrace, illness or old age.
LEATHERFISH: Being herbivorous, Equestrians do not kill fellow herbivores if they can avoid it. Fish, for some reason, are considered fair game. Leatherfish are known for their tough skins, which are cleaned, tanned, and stitched together where leather is required. Their flesh and bones are generally used for either fertiliser or glue.
MOON TEA: A herbal contraceptive, mainly composed of silphium, taken by mares to prevent pregnancy, especially during their estrus period.
MUC: A race of primitive, semi-intelligent pigs that have proven stubbornly intransigent about integration into pony society. Despite researchers not being able to find evidence of prior occupation of the region, the Muc persistently attempt to take over and wipe out the inhabitants of the Shetlands. Witness reports, however, indicate an almost certain link between Them Under Stones and the Muc's predations.
PLOT: Erogenous zones. Generally an Equestrian's genito-anal area. Generally obscured by their tails, and it is considered rude to stare should it become visible.
SALT: Plain salt apparently acts as an intoxicant to Equestrians, possibly by dehydrating the brain. Generally salt licks and salt sellers cut pure salt with sugar or other fillers to reduce the potency. Addicts can be recognised by their profoundly irrational behaviour and extreme thirst.
Salt of a purity above 50% is considered contraband; as such most Equestrian recipes list far more salt than would be advisable in a human context.
Needless to say there is a constant problem with 'lickers' and their supporting criminal activities. (A cup of cooking-strength salt is roughly ten bits, but 'half-cut' goes, according to the most recent reports, for thirty-seven bits per teaspoon.)
SHAGGY: Slang term for a Shetlander.
SHETLANDS, THE: A region several days' hoof travel north of Ponyville, composed of terrain similar to that of Scotland, featuring numerous lakes ('lochs'), mountains, large numbers of standing stones, and often bitterly cold weather. Chief exports are wool, distilled spirits, oats, and the most terrifying hoofball teams known to ponykind.
Names in the Shetlands are a portmanteau of Scottish and Irish Gaelic translations, and the accent ranges from Scotland to Ireland by way of England as well.
STUD: A harem herd. As Equestrian culture became more monogamous, the practice of keeping a stud fell out of favour. Nowadays used as slang for infidelity or a (generally male) prostitute.
SWEETWATER: A brewery town roughly a day's hoof travel north from Ponyville. Renowned for its beer and spirits.
SILPHIUM: A herb probably related to fennel, known for its contraceptive properties. In reality, this plant went extinct due to over-harvesting.
SWORD: A double-bladed weapon held in the mouth. See also WARSHOE.
THANE: Leader of a Shetland clan herd, also one of the ruling council of the Shetlands. This council is known as the Council of Thanes, who convene once a month to discuss matters of the Shetlands, as well as in extraordinary circumstances to select the next Laird.
THEM FROM OUTSIDE: Classically, the denizens of the Chaos from which Equestria has to be protected. Legends suggest that they wait 'beyond the stars', but it is also known by certain defence groups that some of Them are imprisoned underground (e.g. Them Under Stones in the Shetlands.) Basically a Lovecraft/Cthulhu Mythos reference.
THEM UNDER STONES: A subset of Them From Outside currently transfixed beneath the Shetlands by means of enchanted standing stones.
WARSHOE: A metal covering that fits over the hoof and partway up the lower leg, generally featuring an array of spikes and blades to crush and wound the foe. Equestrians are not human, and it makes sense that not only their architecture and furniture, but also weaponry would conform to the most comfortable posture of a quadruped.
Being kicked by somepony wearing warshoes is not recommended.
ZEBRABWE: The distant, southern lands where Zecora hails from.
ZWAHILI: According to my research, "Zecora chants and speaks in an approximation of the Eastern African Swahili language; the crew did not have time or resources to hire a Swahili translator, so they asked Brenda Crichlow to improvise." The passages in this story were constructed via Google Translate.