Stands for Big Titties Girl, or Big Tatas Girls, and usually used to refer to a girl who's name is unknown. For a girl to truly be a BTG, her breasts must be large enough to be in-proportionate to the rest of her body.
Guy 1: Dude, check out that BTG
Guy 2: Wow, those are like bigger than her head.
Usually right after waking up, in a state of about 10% consciousness, when one's imagination creates a fantasy that is more normal than regular dreams, but more odd than regular thoughts or imaginations.
Scott: Dude, i just had a half dream where I had a threesome with 2 chicks.
Steve: What makes that a half dream?
Scott: Well, i was barely awake, and I'm gay.
Steve: Oh yeah, forgot about that.