A person who hacks rides. Slugs are people who need to go somewhere faster than the bus will take them, but don't want to wait for/pay for a cab.
Hacking is basically modified short-range
hitchhiking, and is
accomplished by standing
on the curb, extending one's arm up at an angle, and pointing downwards repeatedly until a car stops. Pricing and destination are then negotiated. May be specific to Baltimore.