birthday suit

noun

1. bare skin; nakedness.

   They were sunbathing in their birthday suits.

birthday suit

noun

1. informal,  a state of total nakedness, as at birth

birthday suit

1. To be “in one's birthday suit” is to be completely naked (as people are at birth).

Word History and Origins

Origin of birthday suit¹

First recorded in 1745–55

Idioms and Phrases

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.