Aphrase which, if used correctly, can be a highly amusing insult. Often used as a come back but can sometimes just be used for the sake of it. It is not a good idea to use it on one of your siblings though as can back fire very easily.
E.G 1 Teacher: John tell me a vital condition needed for successful fractional distillation.
John: Ermm.. Your Mom!
Jack: You suck
John: I suck your mom
E.G 2
John: Hey, Jack!
Jack: What!
John: Your mom!
Jack: What the fuck!
John: Your mom.
Jack: Fuck You
John: Fuck your mom
Jack: Leave me alone
John: Only if you leave your mom alone
Etc...
E.g 3 (At a job interview)
Interviewer: So john what is your main interest
John: Well i am going to have to say...ermm... Your mom
John: OMG! You'llnever guess what hapened to me yesterday!
Jack: What!
John: Your mom!
Passer By: OOH OOH You got owned!!
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