To be ridiculously wet shooting the basketball; used in same context as "swish" or "splash." Can also be used in other context, such as a long killstreak in Modern Warfare 2, and combined with other words to form fantastic contractions.
Here's Bryant, for three...YACK.
I was yackin' on some bitches in M-Dub Deuces last night.
Combinations: yackadaisical, yack in the box, Yackensack, yackity-yack don't talk back, yack my bitch up, etc.