When you leave your computer account (usually at
work) unlocked, someone finds it and they send emails out (usually to a large amount of
people) pretending to be you but saying ridiculous or embarrassing things.
A really good buttshark is one that
people can't tell if the person actually sent it or if they got buttsharked. Another variation is to email one person, usually that person's mortal enemy, and apologize for everything and ask them out for beers but BCC everyone else.