Quote from "Stand By Me" (1986)
When Chris (River Phoenix) and his 3 friends find the lost dead body of a 12 year old boy, he stands up for the body against the local bad-asses. Including his older brother.
"Ass is Grass" is said while they are all in the woods fighting for whose gang gets to claim they found the body.
Charlie: We found him first!
Teddy: Oh yeah, Vern told us how you found him. (mocking tone) 'Oh Billy! I think I just turned my Fruit-Of-The-looms into a fudge Factory!"
Charlie: Thats it! Your ass is grass!