A sexual maneuver: the standing 69. So named because the girl has to cling onto the guy like a possum clings to a tree
Jim: Lauren and I tried to do the possum last night, but my shoulder wasn't feeling good so we just did a regular 69 instead
A man masturbating while hanging upside down.
Fredo: Vic, why are you hangin' upside down right now?
Vic: Because i'm doin' The Possum!