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The goo that oozes out of a man's butt when he sits too long and smells like old pizza. It's made from a butt full of hair, a poop that's wet and smelly, and sweat from a butt that's been working hard.
My uncle's butt has gack in the crack so strong it smells like he ate a whole pizza and then ran a marathon.
I saw my dad's gack in the crack on the couch and it looked like it was about to start a fire.
My brother's butt is so full of gack in the crack, he can't sit down without leaving a trail.