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.