I always understood it to be synonymous with “pimpslap”, and to my knowledge, that is a backhanded slap on the face. Originating with pimps smackin dem ho’s around.
I’d go with mouthbreather’s origin. It’s often used by men to talk about bitch slappin’ another man as well. I don’t think it’s used very much to talk about woman-to-woman slapping.
I did find a webpage called “BitchSlapping - Ike Turner Style” as part of some pseudo-pimp page, but that doesn’t really tell me if it is back hand or not.
Apparently, this is a question that is being considered by the authorities:
Jesse Sheidlower is the North American editor of the OED, formerly with Random House (and Jesse’s Word of the Day), and author of The F Word. I got this quote from this site, and I think it is originally from a NY Times article.