Anyone who drives a public transit bus. The term originated for Toronto Transit Commission bus drivers, though it can be applied to anyone who gets paid 24 dollars an hour + benefits to drive in a circle all day.
The term comes from the fact that their job is so easy a trained monkey could do it.
Bus Driver: "Are you going to pay the full fare?"
You: "Are you going to be here when your schedule says you're supposed to next time, you overpaid lazy piece of shit Bus Monkey?"