Literally: "by the way" - an abbreviated version using the first letter of the first two words and substituting "dub" in play of the double-u sound
Person 1: (walks into house)
Person 2: "Hey, man. What's up?"
Person 1: "Nothing, just gotta take a slam."
Person 2: "Oh, okay. Hey, your girlfriend called a minute ago, b t dub."
A game wherein two gentlemen with erect penises alternate taking turns "thumping" (aka flicking) each other's erection until one goes flaccid.
"Dude, I just got my ass handed to me in thumps during third period."