Ironically, Hootie hoo was first heard on the whitest show on television,The Andy Griffith Show. It was the greeting at the lodge to which Andy belonged. Its current usage would be more surprising if there were not so many other references to the show in urban colloquial language (e.g. Opie)

A hedz up to otha brothaz thet be chillin' in da hood that the popo is out an' prolly aroun' tha corner reddy to bus' yo ass fo smokin' that chronic stack. Made popular in part by Outkast in tha hook from "Slump" off Aquemimi.

several different meanings (varying from it's original meaning as a greeting)
can be shouted out as a means of small celebration, gratification, etc.

Ghetto call to see what is going on or what is lurking in the streets.

that's tha code for tha killahs

The call of the gangster or if u are a faggot ass science teacher what you yell out at the end of spring or some gay shit like that.