KabuSasu is short for the yaoi shota fan-pairing of Kabuto x Sasuke, two character's from the anime Naruto.
Most examples of these would be in fanfiction/Doujinshi that occur after Sasuke left Konohagakure to train with Orochimaru. Some of these are very cute and romantic, where the majority are harsh, and otherwise cruel. Such as, Kabuto would be a sadistic bastard, and when treating Sasuke for an injury of some sort, wouldn't numb the pain and get turned on by his screams.
The PG-13, "fluffy" version of them might be something like Sasuke gets a fever and Kabuto takes care of him, and would finish with the reader (usually a fan-girl) giving an undignified string of "aww, that was so cute!"s, or "Kyaa! So kawaii!s.
KabuSasu doesn't have too big of a fan-base, being far beneath the radar. I have no idea why however, because I find them to be a rather hot couple.
Also, Kabuto is always seme, and Sasuke is the poor soul who is unfortunate enough to be the uke. I think Sasuke is a better uke then most anyways, so I'm not complaining.
-"Now, Sasuke. Stay still, this will only hurt for a second." Kabuto purred, gently running his long fingers over the raven's deep side wounds. Sasuke let out a quick yelp of pain, before hesitantly settling on the table.
A smirk spread onto the silvered-haired man's face after noticing the desperation in Sasuke's eyes. It was funny, seeing the stoic boy force a straight face under so much distress, and Kabuto couldn't help but chuckle softly.-
- Excerpt from "Lost in the Moon's Trill", by me, under my alias.
Random fangirl- "Kyaa!! It's a KabuSasu doujin! OMG!!" That is how the word should be used in the form of a noun.