n. Commuting to work by accepting a ride from a stranger who requires one or more extra passengers to legally qualify to drive in a high occupancy vehicle lane.
Other Forms
In Virginia, many drivers eager to gain access to High Occupancy Vehicle lanes cruise commuter bus stops where people wait in line for a lift into Washington, picking up about 10,000 riders each day, a custom so popular that officials have increased parking-lot capacity at major bus stops. There is even a neologism to describe it, slugging, and a social code for slugs: only the driver may initiate conversation; slugs may not smoke, eat or fiddle with the radio. Some Virginia transportation officials assert that slugging has improved the system. Where early efforts at carpooling failed, the ad hoc nature of slugging has a flexibility that drivers and passengers enjoy. And by reducing the crowding on buses, slugging increases the appeal of public transit.
—Randy Cohen, “The Ethicist,” The New York Times, December 14, 2003
Rick Robillard is a security inspector at Norfolk International Airport, but from 1982 to 1987, when he lived in the Northern Virginia city of Springfield, he was a slug. He admits it.

Each morning, he'd drive to a parking lot near I-95. He'd stand single-file with other slugs and wait for a driver to creep by and ask if anyone was going to downtown D.C. Typically, his wait lasted less than 10 minutes.

The driver sought passengers because he could then legally zip down relatively empty HOV lanes. Back then, each vehicle in those lanes had to carry four people. By picking up three slugs, the driver cut a hair-raising, stop-and-go commute of more than an hour down to an eventless 30 minutes. …

The Virginia Department of Transportation does not openly plug slugging; it fears liability for accidents or crime in the lots. But VDOT loves slugs, who number perhaps 10,000, because each of them represents a car off the road.
—Patrick Lackey, “In northern Virginia, rides are free and everyone wins,” The Virginian-Pilot, May 09, 2003
1992 (earliest)
Maybe it means America has gotten down to business: Baby boomers who hitchhiked off to adventure in their youth now catch rides from strangers to get to their 9-to-5 jobs in Washington.

And hitchhike-commuting, known as slugging, is an increasingly popular way to get to work.

Slug lines form early, before the winter sun rises.
—Doug Graham, “Slugging it out in thick traffic,” The Washington Times, February 25, 1992
The term "slug" itself did not derive from the word that means mollusk, as some people think. Instead, the term appears to have originated from bus drivers as a derogatory term.

The story goes like this. Bus drivers had always been warned to be aware of counterfeit coins (also known as slugs) from people trying to pass off this fake money in the coin collection tray.

When slugging was in its infancy, commuters stood at the bus stops, waiting for a driver to pick them up. Bus drivers, thinking these people were waiting for the bus would stop to pick up the passengers only to be waved off, frustrating many of the drivers. As this event became more and more frequent, bus drivers began recognizing the real bus riders from the fakes. Because the people weren’t really waiting for the bus, drivers began to simply call them "slugs." This definition seems to make sense because these people weren’t real bus riders or even real car poolers in the usual sense of the word. They were, just as the name implies, counterfeit riders or slugs. Hence, the term was born.
—“About Slugging,”, February 12, 2003