Colloquial short for "Rigglebop"
A nasty, disgusting monkey who's single mode of sustenance is eating his own feces and drinking his own urine.
It is thought that the feces he ingests was, once upon a time, originally banana, but since it has been eaten and re-pooped out over and over again, it has been rendered an unrecognizable brown/grey sludge, known by experts on Rig-ology as "sike juice" (as in, recycling juice)
Rig Rig is so bacteria-ridden, to touch him will likely lead to any number of nasty germ infections that commonly occur in 3rd world countries. To taste his "sike juice" is a death wish ... it's so disgusting, no one has survived sampling it and lived to describe the taste. Which is probably really bad.
Me: Get that nasty poop-eating Rig Rig out of here!
Or, another ...
Me: Eww, be careful not to get any of Rig Rig's "sike juice" on or near your person!