Where a driver has more of a tan on their forearm, according to which arm they put out the window of the vehicle they drive in, which receives more sun than the other one that is inside the car. It was originated from the truck driver as they drive long roads and long hours.
Johnno: I've got a big trucker's tan on my right arm.
Jenny: I saw, it looks odd.
A sunburn or suntan focused on the left arm. It is caused by the left side of the body being the only half that gets exposed to sunlight while driving. Presumably, it would be located on the right side of the body in countries where they drive on the left side of the road.
I had a trucker's tan after driving for 10 straight hours on a sunny day.