A man who shaves small animals and fires the fucking thing up his ass using a tube of some sort. The animals can be any of the following: gerbils, mice, hamsters, rats (either sewer or store boughten), beavers, muskrats, small dogs, large dogs or pretty much any other animal you can think of shaving.
Gerbilling is an odd practice of inserting a small rodent (mouse, often) into a condom which is then inserted into the anal cavity, where the dying convulsions of the poor animal are said to produce a pleasurable effect.
The term is derived from a combination of the two practices.