"Don't be a tun tun" - Don't behave like a cunt
"Look at that tun tun over so" - Look at that pussy/cunt across the street "She has a fat tun tun" - She has a fat vagina
A phrase said while in an intoxicated state, usually from marijuana, when one is either confused, excited, or just wants to be a dick to his or her friends, which are usually intoxicated as well.
There is no comeback to it, except for "fru frufrufru", and is usually used to win an argument.