But we sittin' on your grave site: overkill

from Royce Da 5'9" (Ft. Eminem) – Writer's Block Lyrics on Rap Genius

Meaning

You’re already dead (“on your grave site”) and Royce is sitting on it. It’s overkill because they’re messing with you after you’re already dead!

He’s also, in a literal sense “over” the kill in that he’s sitting on top of it (T-Bag)…

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