This is a phrase used to express that ur not gonna do something you dont wanna do. This happens to be the bestcomeback to anyone asking you to do something for them; It lets the person know that doin a favor for them is not "good" for you.
Ruby Tuesday Cook: "Hey man can you prep me some Tomato Mozzarella"?
A more polite way of saying "I don't give a flying fuck" in Instant messaging. Although McDonald's new bullshit slogan uses the same lack of punctuation,nah im good is not used by closet homosexuals like Justin Timberlake.
0TTR: It's PEANUT BUTTER JELLY TIME! *link to eBaum's world*
mrterryakiman: nah im good