"Slim Shady? Shit sounds like a fable to me"
'til he jumps out of the fuckin' toilet when you're taking a pee!

from Eminem – Underground Lyrics on Rap Genius

Meaning

People didn’t expect Eminem to be back, and sure enough, his alter ego Slim Shady will jump out of the toilet while you are taking a piss.

This line is probably inspired by an incident where Eminem punched a fan for asking for an autograph at a strip club urinal (Eminem gets violent in the restroom!)

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