Acronym for Imma Bitch Slap Yo' Ass. Generally used in situations wherein expressal of such feelings would be looked down upon, such as a school, in the workplace, or in public. Usually conveys truthful meaning, rather than just a joke.
Carolyn: WTF Rema?! Where is the work I asked for?!
Rema: Oh man, sorry, I was so shitfaced last night, I totally forgot to write that up.