a person who tells on you, rats on you... snitches. no matter what size this person is pr what their physical strength, this person is considered a bitch because they are a snitch. the "ass" part of the phrase comes from the fact that being a snitch and a bitch make you an ass. hence the term snitch ass bitch
person 1: "yo, i got fired from the convenience store.
Bobby told the manager i was stealin zig zags."
person 2: "seriously? what a snitch ass bitch"