Generally stands for "yeah nobody cares." It is often found following statements of which no one on the face of the planet would care about or excessive e-peen. For those not into the whole brevity thing, "yeahnobodycares" may also be used. Capitalization doesn't really matter (i.e. ynbc, Ynbc, YNBC, yeahnobodycares, YeahNobodyCares).
Guy #1: Wow, I just did something awesome!
Guy #2: ynbc