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1 definition by AFAN

 
1.
I understand it to mean "error" or "screw-up" or "bad behavior". An abbreviated informal apology.

While some believe it to be a misinterpretation of the older phrase My Bag it is clearly it's own slang with its own origins and meaning.
While playing pick up basketball a player who just made a mistake would say "My bad"
by AFAN July 13, 2005