He ain't had him a buzz like this since the last time that he overdosed
Quadruple entendre people! The buzz he’s talkin about was:
his buzz cut when he first blew up.
the amount of publicity he had when he almost overdosed
the actual buzz the fans are producing for him.
the buzz of the defibrilator used to revive him following an OD.
A buzz for the average person is the result of a couple drinks or a puff of a joint—even a cigarette for some.
This line suggests that a buzz for Eminem is an overdose (in parallel to over-performing). Hence: Eminem has an outrageously high drug tolerance (as opposed to other rappers who get a buzz from less good performance).
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