The "Rude witty" is when, in order to end a conversation or argument, one pulls their pants down, turns around, and bends over to reveal their buttcheeks, after which they start to drum the William Tell overture on both cheeks.
There is also the extreme edition, which is where one farts the brass sections of the song, and manages to finish the entire piece before the other person walks off.
Right when my boss was about to fire me, I broke out the full extreme edition of the rude witty, and left the office with dignity.