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2.
It is a combonation of urination and defacation in the same restroom visit. Derives from from the words number 1 and number 2.
James: Hey Travis you were in there awhile. Did you do a number 2?

Travis: No man, a number 12.

James: Why do you give it a special name? Everyone pees when they poop most times...
by Wannnabe55 May 20, 2009
 
1.
slang for head, dome, oral sex, or pp in the mouth. Originates from mcdonald's and their #12 Dome piece chicken
Yo nigga i got number 12 all night long
by elmoslicer January 28, 2008