(British) A girl's breasts. It is a play on words. A "bouncing betty" is a name for a certain type of booby trap. Accordingly, it refers to boobs - breasts.
She reluctantly pulled down her bra to show her bouncing betties on the topless beach.
An urbansport in which involves throwing objects (preferably bouncy balls) at people, signs, other vehicles and then yelling a witty catch phrase out as you drive off. NOTE: Best done with one person as the driver and another in the back seat doing the throwing and yelling
Last night me and the Yugo went out bouncing. We hit this bogan and he ran after us yelling, "Come on ya cunt!"