The hair that covers your butt-cheeks.
May look disgusting sometimes, but still rocks when making fun of it.
Also good for showing your lack of respect for another person.
"Hey dude, let's go and set my buttfluff on fire."
"Shibby!"
or
"You're a dick man."
"Ah touch my buttfluff, ass eyes."
Short-hand for the word floccinaucinihilipilification, meaning the estimation of something as valueless (encountered mainly as an example of one of the longest words in the English language).