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1.
A casual, slang expression which can be used in a variety of situations. It can be used as a response to a person's rude remarks, behavior, ineptitude, or unreasonable demands and favors.

Similarly, It is another way of saying don't worry about it, it's not your fault, you're fine, no thanks, or that's OK.
When someone accidentally bumps into you at a crowded bar:

Person 1: "Oh my bad man, I didn't see you there."

Person 2: "Nah you good."

When asked to do a favor:

Mom: "Honey, can you take out the trash? It's overflowing and there are ants everywhere"

Son: "Nah you good."

If someone tries to cut you in line at a fast food restaurant, throw out a stiff arm and say "nah you good."
by PWoo August 24, 2009