I don't even know. Nothing on the internet has clarified this for me.

A female, usually a teenager or young adult, who, when insulted, will reply verbally. This is typically achieved by insulting the original insulter (otherwise known as 'hollering back', shortened in this case to 'hollaback').
Ooooohhhh..... I get it now.
Honestly, the context in which they used it really confused me.

Urban Dictionary? o:
In the context of the song it has a different meaning, a Hollaback girl is also someone who just hooks up for sex or lets herself be used for sex.
When the dude gets up to leave he'll say "I'll holla" (I'll call) and when he does, it's just for random bootycalls, never a meaningful relationship despite whatever the girl thinks.
wow that is interesting... one would also call a hollaback girl a skank, ho, whore, or more commonly used a slut
Working on filled wardrobe achievement. Almost at 12k!
Wow...didn't know thatO.o I was always confused by that wordXD

I found that icon awhile ago and it made me laugh.
I thought it was something made up by Gwen Stefani as well. Even though I now know the meaning I doubt I'll actually use the word ever.
wow! =D
i could have lived the same as i do now without that piece of information....
^^;