What is Donut Froster?
1.
Someone who jerks off in the bathroom and gets cum on the toilet seat.
BOB: I never invite Frank over to my house any more.
JOHN: Why?
BOB: Cuz he's a donut froster.
See
2.
a person who goes out of his way to put a toilet seat down so he can piss on it - so the the next human being can slip and slide.
Johnny entered the stall with a giggle, unzipped his pants, kicked the seat, and shot his hot yellow sauce all around the horse shoe.