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.
A tan where the tan lines are obviously caused by wearing normal clothes outside in the sun for long periods of time, instead of wearing swimwear or more suitable clothes for tanning.
Dude, your chest is so pale compared to your arms. And why are your knees so red? Get rid of that farmer's tan before you come to the beach again, it looks ridiculous!