Highly bastardized, this phrase originally was a shortened form of "Who Got That?" That usually meaning weed or some other friendly substance.
Somebody or Somebodys, after calling people and asking 'Who Got?' so many damn times simply began employing the phrase to mean almost anything, including 'What's up?' , 'Excuse me?' , 'Hell yeah!' etc...
Sometimes it is still used to find out who got.
Guy looking for that: "Yo man, all I wanna know is... Who got?"
Guy to other guy, upon meeting: "Who got!?!"
Guy 1: "I'm hungry, let's go to pizza hut, pizza hut's the shit!"
Guy 2: "Who got!"
What you ask someone when you're trying to find out who got that.
Ian - "Whooooooooo gaaaaaaht!?!" (who got?)
weed man - "I got that"
Ian - "Whooooooooo gaaaaaaht!?!"
co-workers (in unison) - "Whooooooo gaaaaaaaaaht? Ut!"