| bare skin; nakedness: They were sunbathing in their birthday suits. |

| birthday suit n. The state of being nude; nakedness. |
To be “in one's birthday suit” is to be completely naked (as people are at birth).
birthday suit
Nakedness, as in The doorbell rang, and here I was in my birthday suit. In 18th-century Britain this term originally referred to the clothes one wore on the king's birthday. Later it was jocularly transferred to bare skin, alluding to the condition of a newborn baby.