Ass so mean that I’m always on her bad side

from Joe Budden (Ft. Lil Wayne & Tank) – She Don't Put It Down Lyrics on Rap Genius

Meaning

He’s using wordplay to say that he likes looking at her from behind.

To be on someone’s “bad side” is an idiom for when someone is criticizing you harshly.
In this case, her bad side is her “backside”. Get it?

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