A (usually intense) fight between two people, usually women. Two or more bitches fighting, most likely verbally.
The first section of this compound word (bish) is a commonly used form of the word bitch, which pretty much means the same thing. Spelt properly, it is a bitchfight.
Marco: Hey man, did you see Sarah and Kate yesterday? It was a total bishfight!
Ken: Whoa, you mean I missed it? Must have been intense, knowing those two.