An unlicensed or on-the-side mechanic that you go to when you can't afford to go to a real mechanic. Often used when the job is simple, or you no longer want to dump money into your very old car.
Can also be applied to a halfway knowledgable person in any field (ie: computer repairs, home repairs, gender reassignment)
"I took my car to this grease monkey I know and he screwed up my tranny. That'll teach me to stop being cheap."