Combination of the phrase "your ass" and the slogan "verb, its what you do" thus meaning their ass is what you "do." Often used as an insult or inside joke.
Old skool dissing. When your mum and your dad have been used, drop the bomb that is 'YOUR GRAN'!
Kid 1: Your mum
Kid 2: Your dad Kid 1: Your gran
Kid 2: Your uncle
Kid 1: Your aunt
Kid 2: Your sister
Kid 1: Your brother
Kid 2: Your grandad
Kid 1: Your cousin
Kid 2: ...Your step-dad Kid 1: What? Thats shit you dickhead!
Kid 2: ...Your mum!