Whale Back is when a female's butt crack starts much lower below the back, so when she bends over it looks like the back of a whale. This usually results in a very small unimpressive or nearly non-existent butt.
Friend 1: "Hey, you should ask out that new chick who's been hanging out with your sister. She's hot."
Friend 2: "Hell no, she's got whale back. I like my women with fine asses."