the act of publicly exposing the male genitalia
"Did you see me hang dong on the bus on the way to thunder gun?"
"Yeah, it looked like a piece of chalk and a penny was hanging out of your pants..."
To expose one's penis
Charlie: "It's like the biggest movie in America."
Dee: "Plus, I hear the guy hangs dong and I'm very interested in seeing that."
Do you like to hang dong on the trolley?