It's when you been sitting for a long periods of time(in a warm climate, warm room, etc...) and acne develops on your ass.
"Dude, I finally finished that game!" said C.
"Oh, yeah? How long did it take?" asked T.
"Just over 9 hours,...Oh Shit!" replied C.
"What?" asked T.
"Damn it!...I know I'll have zitass tomorrow!" said C.