Typically a scummy oil industry tradesperson, the bottom of the barrel on an oil rig. The toby/gopher who gets all the crappy jobs.
The term is now en vouge and tradespersons are lowering themselves to the term as a form of rebellion.
Rigpigs typically drive stupidly oversized 4x4 trucks, called RigRockets, usually decked out with ALL the plastic wind deflectors and fake chrome doo-dads they can find at JC Whitney or Canadian Tire. They also tend to drive aggressively, battering their way through traffic with little to no regard to anyone who drives a smaller truck or makes less money.
Joe drives his rigrocket through traffic, aggressively tailgating until other drivers move out of the way. Joe's ugly APC chrome trim glints garishly in the sun and his farmer-tanned elbow juts out the window in a challenging gesture while he listens to crappy grind-core.
Joe is a rigpig and damnit, he's proud.