It's the body bag game, bitch, I'm supplying coffins

from Eminem – Legacy Lyrics on Rap Genius

Meaning

Em is alluding to the infamous Eagles vs. Redskins game that took place in 1990 where nearly the entire Redskins roster were getting injured and carted off the field.
http://www.youtube.com/watch?v=IInU3PgwFVc

Which is pretty much what Em does when he is on the mic. He sends MC’s into body bags.“Coffins” is a play on words

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