A word with no true definition as a noun, but is generally used as a word to throwpeople 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'?!"
A phenomenon in which a man sits in a certain way that allows for his balls to droop down to his buttcrack, so the scrotum comes into contact with the edges of the anus.
Dude, my buttsack is so intense right now, that if I farted, my balls would blow up like a parachute.