'Trucker's tan' is the name for the tanning pattern where one arm, below the sleeve, is visibly sun tanned darker than the other arm (and usually the rest of the body). It's called this because long-haul truck (lorry, for those of you from the U.K.) drivers tend to get a deep tan in this pattern due to sunlight from the side window striking their arm (in the U.S., the left arm). It generally happens to anyone driving on a long trip on a sunny day, to some degree.