When I'm not even in my harshest
You can still get roasted cause Marsh is not mellow

from Eminem (Ft. Lil Wayne) – No Love Lyrics on Rap Genius

Meaning

A “roasting marshmallows by the campfire” pun: Marshall Mathers (Eminem’s real name) is not mellow, so he’ll “roast” (slang for “criticize/make fun of”) you if you make him angry; it’s also just another way of saying he’s “on fire”, even when he’s not at his best.

Taking the lyric less literally: even when he isn’t trying his hardest, Eminem can still take anyone apart using only words.

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