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
with the inserted hand while shouting at the top of their voice "UMBRELLA STAND!!!".
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!