A/N: So I saw someone shared a headcannon with me and I just had to write this. I had been planning it for awhile now.
I've always loved how the fandom interprets the Skelebros as really close; Paps carrying Sans around, their joking and teasing with one another. All the fluffy familial feels and hugs :3.
But I've also always loved the idea that Sans doesn't like being close with anyone but his bro. This then developed into the idea that maybe Sans doesn't like physical contact from other people - of course post-True Pacifist he gets used to, and lets, Toriel, Undyne, Frisk and maybe Asgore and Alphys hug him and such but before then and in general he's uncomfortable with it. Obviously he gives handshakes (it is a social norm. One that I don't always follow*cough*) and Dogamy mentions something about expecting a pat on the head from Sans, so he's probably okay with smaller things like that as long as he is the one initiates it.
Also I really like the idea of Undyne and Sans reaching some level of friendship.
DISCLAIMER: All hail the Annoying Dog.
Space
She probably wouldn't have noticed, in all honesty, if it had been a subtle reaction. But she knew Sans - even just a little bit, even as Papyrus' lazy, laid-back brother - and he wasn't one to let his emotions manifest as a physical reaction.
So, when she pat him on the shoulder after her and Papyrus' training session (Sans had come along for moral support, though Paps kept trying to cajole him into sparring) Undyne was surprised at the noticeable flinch, his bony shoulder tensing beneath her touch, and the fleeting moment where his eye sockets had widened. He quickly composed himself (though he was still somewhat tense beneath her hand) blinking at her innocently. " 'Sup Undies?"
She decisively ignored the nickname ("Really Sans?" "What?" "I like it Undyne!") regaining her thought. She gave a shark-toothed grin. "Was just gonna say Punk, maybe you should join us next time! Paps told me you taught him his magic."
He shrugged. His smile could almost be considered uncomfortable, though due to what Undyne didn't exactly know. "Yeah but nah. Don't want to ruin my image." He gave a wink and chuckled.
So suddenly, Undyne wasn't sure how it happened, he was out from under her hand and slouching casually, a step or two away. "Welp I'm gonna go get some supplies. You guys were wanting to do Lasagna tonight, right? I'll go get the stuff." And with that he was cantering away at his usual slow, steady pace.
She was frozen for a moment, then cupping her hands over her mouth she yelled, "You don't even know what we need!"
He responded without turning, his voice easily heard but not actually yelling. "Text me the list."
Undyne watched as he receded, curiously.
"Sans doesn't like being touched."
Undyne whipped around, eye wide. Papyrus stood next to her, eyes glued to where Sans went. How had he even snuck up on her like that?
His voice had been almost … meek. Quiet not necessarily in volume but in tone. His gaze shifted to her face, as he repeated. "He doesn't like to be touched."
Undyne blinked at him, taking a minute to get what he meant. Then it clicked that her friendly little shoulder-pat had made Sans uncomfortable.
She hid a snort behind her hand.
Papyrus' gaze sharpened but then immediately softened, though he still crossed his arms. "It's not funny."
She quickly composed herself, still smiling in amusement. "No you're right Pap; I just wasn't expecting Sans to be all anti-touch. He's, like, the friendliest guy around! It's a little odd, y'know."
He did quirk a smile at that. "Yes. My brother is very friendly. But he's never really been okay with people touching him - other than me of course!" His permanent grin held what Undyne had come to recognize as a frown. "I wonder why … ?"
Undyne didn't understand why Sans did half the things he did, so she quickly amended with, "That's probly because you give the best hugs, Paps - and no one else can compare."
That sunny smile blossomed back awfully bright. "Of course! Nyeh heh! Why didn't I think of that! The Great Papyrus gives the best of hugs!"
As usual, the conversation had been derailed from Papyrus' worry, to flaunting his greatness, and continued on that train of thought as they got the kitchen ready to cook.
Though Undyne did make a mental note to learn more about Sans in the future.
He was her best friend's brother after all.