Another word for asshole, wanker, twat, pussy etc. Mainly used in Britain as an insult towards people, but also said in countries such as Australia and Canada.
A possum is something at which you would usually refer to as a piece of s***. The term, piece of s***, is derived from POSsum. Being possums here in Texas are such nastylittle rodents (well, they are in Houston =\), Possums are dubbed as a piece of s***.
"Hey check out that old computer sitting there in that pile of trash. Tis such a possum."
"So this (computer) is a breakdown (to be disassembled)? Well then, grab that possum by its tail (by the power cord) and load it in the truck. :B"