Gwen Stephani (she was in No Doubt, and looks like the girl from Tank Girl) has a song called Hollaback girl.
In the new Kesha song, (name escapes me), she suddenly shouts ‘Holla’.
Now I assume that Holla is the bastardisation of holler, but I’m not sure as to what it means exactly, or what a Hollaback girl is, no why it is a negative thing.
Any thoughts?
EDIT: I am familiar with the phrase, ‘I’ll give you a holler’, but I don’t think that’s it.
Urbandictionary.com is the closest thing we have to a teenage translator. It appears that Ms. Stefani is about lay the smack down.
Here’s the current scholarship on the Gwen Stefani lyric. This page addresses the latter term.
ETA: Crap.
Are you sure the word shouted is “holla” and not “hola”. Have you seen the official printed lyrics for example? Hola would seem to be the more obvious choice of word used.
Hella holla! Word to your mother! 
Sounds like Holla.
“Holla” is an exclamation used to indicate affirmation with something that just happened or was just said, or can be a general exclamation of excitement. Why do you think something saying “hola” would be so much more likely? How often to songs say “Hello” without any other accompanying lyrics?
In Hollaback Girl the “h” is clearly pronounced. If it were “hola” the “h” would be silent and it would be pronounced “ola.”
In literature and music? All the time:
Hello
interjections / nouns hello, hallo, hullo [həˈləu]
a word used as a greeting, to attract attention, or to express surprise Say hello to your aunt; Hullo,’ I said to myself, `What’s going on here?’
3. Used to express surprise or to register an unexpected turn of events.
1838 J. C. NEAL Charcoal Sketches 157 He stumbled over something which lay like a lion, or a bundle of wet clothing, in his path. ‘Why, hello!{em}what do you call this?’ 1854 Knickerbocker Apr. 349 Pshaw!..your fingers must be very tender. I’ll bring it out for you. Hello! it is rather strong. 1908 J. LONDON Martin Eden i. 2 ‘You mustn’t be frightened at us. We’re just homely people-Hello, there’s a letter for me.’ He stepped back to the table, tore open the envelope, and began to read. 1922 ‘R. CROMPTON’ Just{em}William v. 116 ‘Hello!’ said Henry, surprised. ‘That’s not taken long.’ 1967 Listener 5 Oct. 427/2 ‘Hello,’ I thought, ‘Now she’s overdoing it.’ 2007 Austral. Mag. (Nexis) 2 June 43 ‘Hello,’ I thought. ‘Something’s different.’
OED
Those are all in the context of a greeting or otherwise couched in context. As per the OP:
This to me suggests a different use than the ones you’ve mentioned. I also note that most of your cites are of literature from the 1800s and early 1900s. What bearing would cites from a dated era and separate medium have on the use of language in current popular music?
“Holla” makes perfect sense. And it does come from “holler”, which is often used in my part of the country to refer to yelling across the room. “Holla!” is quite common with certain types of singers in concert, used to ask for audience feedback. They’re saying “Yell back at me.” And it’s morphed into a statement of empowerment.
As for a Holla back girl, it’s similarly based on “holler.” To holler-back at someone meant to respond to someone when they yelled at you from a distance. This was then used to refer to calling someone back on the telephone. You would say “Holler back if you want to hear from me.” As it morphed into holla back, it became more used for calling someone back after a sexual liason, even after you’d broken up. A hollaback girl became someone who would go back with the guy no matter what.
The Kesha song is called ‘Blah, Blah, blah’.