A bottom-feeder, parasite, low-life con-artist type. They are usually found looking to cheat/beg others out of small amounts of money, or freeloading. May also be a drug addict, alcoholic, homeless, dirty, or a criminal, but not necessarily.
Often heard being used by old school New Yorkers.
Shit, there goes that f*ckin' skel, Stucky. Let's cross the street and hope he don't see us.
Back in the day, the Port Authority bus terminal used to be full of skels.