Swearing isn't good for you. This is how Sherlock had been raised. 'The use of profanity is the attempt of a feeble mind to express itself forcibly.' His Father had repeatedly passed this judgment whenever a colourful, layman form of communication was overheard.
His Grandmother reproached swearing as simply 'the weapon of the witless'. And, as Sherlock loved his Grandmama, and feared the power and intellect of Mr. Holmes, the senior, he never questioned their interpretation of language.
Mycroft had often called the bluff of other students when they were at school. 'When a man uses profanity to support an argument, it indicates that either the man, or the argument, is weak - probably both.' While Sherlock didn't really respect his brother, he gave him credit for standing his ground with the idiots who shared their tutelage.
All John knew about swearing was that he preferred not to use it, if he could help it. He knew ALL possible combinations, of course… you don't spend time in the army without acquiring THAT education.
So it was with complete surprise one day, when he entered 221B, that he found his flatmate reciting…
"Bang. Bastard. Biggy. Bitch. Blast. Bloody. Blow. Bollocks. Bum."
"What are you doing, Sherlock?"
"Expanding my vocabulary, by reviewing profane terms, in alphabetical order."
"Bullshit!"
"Hmmm… missed that one. Bugger."