Just another slang word for "nigga"/"negro" in the "homie"/"buddy" sense, one of the hundreds of random and innocuous corruptions of the original word.
noun, plural necro-bitches,
1.) Derived from the words "necro" meaning dead, and "bitch" meaning modern day slave or excessive whiner; An un-dead whiner.
2.) Female zombie.
Jonathan yells at the zombies banging their heads against the wall: "Hey, necro-bitches, munch on this!"