An act whereby one runs up behind a friend (or stranger) with a clenched fist, rams it up their bum hole lifting them up above their head with the inserted hand while shouting at the top of their voice "UMBRELLA STAND!!!".
Mostly used on enemies as a form of embarassment, as an extreme form of foreplay or on friends as a hilarious practical joke.
Hey Pete, theres that girl you fancy. Asking her out is SOOO last season, if you want my relationship advice you should umbrella stand her!