tan lines

Usually the mark of a person who wears a bathing suit while in the sun. On a nude beach such people are called cotton tails.

For a naturist this also implies that the person is not into body acceptance.

Tan lines are really any line defining the parts of your body exposed to sun and the parts that aren't. The only tan lines I have are on my arms, about elbow-level, from where my shirt sleeves end.

But tan lines do look pretty goofy when all of someone's body is deeply tanned except for the little patches that were covered by a bathing suit.