Student or Learner
A: You probably don't have a hair on your body, do you?
B: Just back your punk-ass up.
What do both sentences mean?
A: if it's an idiom, no clue.
B: to back up = to make a reverse move. If you were going north, then you stop and come back south.
You cousin comes to you to talk shit, noting worthwhile hearing, and he should be cleaning the bathroom:
Hey, back your fat ass up and go clean the bathroom.
[I am not a teacher]