The last dick picked in an assortment of dicks. Similar to Bingo it's the last pick, which consequently, only 1 person is proud of. In this case it's likely their mom or some random old person.
Caroline: "Dammit, I don't think anyone can make it this weekend."
Ray: "Don't worry I can still make."
Caroline: " Well whoopee for you, bingo dick..."
A fun little game males can play with each other. It originated from the old question a male asks "What's the capital of Thailand?" and upon the other male not knowing, the questioner yells "BANGKOK!" and hits the other guy in the balls. (Bangkok=Bang cock; get it?)
Bangdick, however, is not just limited to the questioner responding with Bangkok. Originally, Bangdick was the only proper response, then it switched to any random world capital, and now it is even acceptable to just be literal and say what you are doing (this also adds humor). Really, you can say just about anything, so long as it is funny.
Guy 1: What's the capital of Thailand?
Guy 2: I don't know.
Guy 1: BANGDICK!
(Guy 1 hits Guy 2 in balls)
Guy 1: What's the capital of Thailand?
Guy 2: I don't know.
Guy 1: TOKYO!
(Guy 1 hits Guy 2 in balls)
Guy 1: What's the capital of Thailand?
Guy 2: I don't know
Guy 1: PUNCH IN THE BALLS!
(Guy 1 hits Guy 2 in balls, hard)