You don't really wanna get into a pissin' match with this rappity-rap
Packin' a mack in the back of the Ac

from Eminem – Rap God Lyrics on Rap Genius

Meaning

The last thing you want is some type of pissing contest with Em.

The second line is a reference and homage to the late legendary rapper Big Punisher. On his album “Capital Punishment” there is a funny skit where Pun explains that he “shot the place” because he was “packin' a MAC in the back of the Ac'” Or in other words, had stashed a MAC-10 machine pistol in the back of his Cadillac.

http://youtu.be/vgP6579rOwI?t=24s

The words ‘Packin a Mac in the Back of the Ac’ has been dropped by countless rappers to reference Big Pun’s excellent lyrical skills and sense of humor.

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