My job here isn't done

from Eminem – Rhyme or Reason Lyrics on Rap Genius

Meaning

Some critics have claimed that Eminem’s time has come and go, and it is time for him to put down the mic permanently. But Em still feels he has work to do and isn’t going to stop rapping until he stops enjoying it.

He speaks on retirement briefly in an interview with Zane Lowe:
http://www.youtube.com/watch?v=DOM2Afq_sTY&t=7m32s

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