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1.
Means "I don't know". A shortened form of "IDK, my BFF Jill?" from the Cingular commercial.

Mom: "Beth Ann!"
Beth Ann: "WU?" (What's up?)
Mom: "Your cell phone bill is what's up. All this texting!"
Beth Ann: "OMG, INBD!" (Oh my God, it's no big deal!)
Mom: "It is a big deal. Who are you texting 50 times a day?"
Beth Ann: "IDK, my BFF Jill?" (I don't know, my best friend forever Jill?"
Mom: "Tell your BFF Jill that I'm taking away your phone."
Beth Ann: "TISNF!" (That is so not fair!)
Mom: "Me paying this bill, that's what's...S...N...F!"
Person 1: Dude, where'd you get those jeans?
Person 2: Uh, my BFF Jill.
by Elvi December 31, 2007