The expression of which your ass is grass comes from if you hear this you better get out of town because you're likely to get shot hung or end up with your head on a stick
Idiot1: oh dude I am so dead
Idiot2: yep that's quite a bit of a fender bender there
- idiot1 gets home-
Idiot1's roommate: heard you my moped just want you to know ass is grass and I'm the lawn mower
Idiot1:* shit himself*

And that my assholes is how the term:
Your ass is grass and I'm the lawn mower
Turned into: your ass is grass
by ಠ_ರೃ 1997 June 27, 2018
Get the your ass is grass and i'm the lawn mower mug.