The driver who chooses to stay in their lane with no signal, or intention of getting over, even though they have had ampel warning the lane is ending and they need to merge. "Lane lagger" can also be applied to the inconsiderate driver who knowingly gets into the lane that's ending during heavy traffic, because they'll get ahead a few cars since another driver will have to let them in.
Although anyone can be guilty of lane lagging, it's most often observed happening to drivers of high-end cars because sense-of-entitlement usually comes with the purchase of their vehicle.
"That lane lagger wants in front of me but doesn't have the courtesy to signal!"
"Officer, the BMW was lane lagging and merged right into my bumper at the last second!"