Tub means tall ugly blonde. It's an acronym. There are a lot of these out there. Especially the ones that from the back look cool, and you're like, hey maybe I'll pick up that hottie. Then she turns around and you're just straight outta luck son.
Guy 1: "That blonde over there looks awesome from behind - I'm trying to get a look at her face"
Guy 2: "Yeah man, hope she doesn't turn out to be a tub"
Guy 1: "Damn she turned around and she's totally a TUB!"