To stick ones finger ,usually in the form of a sneak attack, in someone's ass crack when it is exposed above their pant/underwear line. While doing so scream "Buttack!!!"
As Emmett leaned over exposing his plumbers crack Jason exclaimed "Buttack!!" While sliding his pointer down his toosh.
When a person is born with the parts to form a butt (butt cheeks) but no flesh ever properly forms in it, so that the prson doesn't have the natural roundness of a butt and are therefore forced to sit on the extension of their thighs for lack of a natural butt. Or in simpler terms- lack of butt meat.
Hold on, is that Crystals's thighs or just her vestigial buttsack that's creeping from the top of her jeans when she bends over?
A word with no true definition as a noun, but is generally used as a word to throw people off when a question is asked that the person doesn't want to answer and/or might be dangerous to answer.
Woman- "Do these pants make me look fat?"
Man- "Buttsack."
Woman- "What?"
Man- "Buttsack."
Woman- "...What were we talking about, again?"
OR
Girlfriend- "Do you love me?"
Boyfriend- "Buttsack."
Bystander- "...Did he just say 'buttsack'?!"