The concept is ingenious: 1) Form a line of passengers near the freeway, 2) hitch a ride from a car passing by to make a total of three or more passengers, and 3) take a ride on the freeway in the High Occupancy Vehicle (HOV) lane and get to work quicker.
If you do this, you're slugging. What sets it apart from other forms of hitchhiking is that both the driver and the passenger benefit. The drivers may benefit from the slugs just as much as the slugs benefit from the drivers.
While I was searching for the origin of the word "slugging", I came across this great online resource for slugging has an about page that covers the history of the phenomenon. According to this source, the word "slug" originated from toll booth attendants who were warned of fake coins from commuters called "slugs". Then, in the 1970s, when people started to form lines to hitchhike and take advantage of the new HOV lanes, buses often stopped to pick these people up. Annoyed by the false bus riders, bus drivers became better at distinguishing between real bus patrons and the fake ones, then deemed "slugs".
Again, I was delighted to find such a thorough source on this neat term for Washington, DC. Please check out the about slugging page and learn more!