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A euphemism for self gratification, particularly for females. 'Two' meaning 'using two fingers to pleasure oneself' and 'bing' meaning 'if you hit the spot just right, it goes BING!' (as in, a buzzer in a game show).
"Are you going twobing again?"
"Shut up! I have needs too!"
by FileCurropted September 12, 2007