Trucker Slang. Referring to the right lane or slow lane.
That asshole needs to be in the Granny Lane. He's holding up traffic in the Hammer Lane.
The right lane of any roadway having more than one going in the same direction is designated as the slower driver lane and is intended for drivers that are not in a hurry and want to do the posted speed limit or are exiting the roadway and need to reduce speed while getting off at the ramp or are unsure of where they are and need to drive slow as they look for signs.
Some drivers need to keep up with the flow of traffic or they should stay in the grannylane.