Origin: When a junkie (drug addict) buys heroin from an unscrupulous ice cream van, the height of the service window denotes that he adopts a dinosaur-like stance in order to achieve his drugs into his filthy hands.
This, in turn, allows the phrase Scumbagosaurus to be used to describe any given junkie, as they will likely have had to adopt this posture in the past, or in every single day of their existence.
"DAD! What the hell is that on top of that bin spitting blinding-glue at those pensioners whilst pathetically roaring?"
That, my son, is a Scumbagosaurus. He'll be dead in five years. Lunch?