Why 'Slugs'?
Slugs are people who line up for a free ride at designated spots such as commuter parking lots and street corners. According to Slug-Lines.com, the term probably came from bus drivers, who would be on the lookout for counterfeit coins (also known as slugs). Slugs used to stand at bus stops, waiting for cars to pick them up. When bus drivers would stop, the commuters at the stops would waive them off. The bus drivers soon began calling the "fake" bus riders "slugs."
Source: https://www.npr.org/templates/story/story.php?storyId=90700559