| birthday suit | |
| —n | |
| informal, jocular a state of total nakedness, as at birth | |
| a children's mummer's parade, as on the Fourth of July, with prizes for the best costumes. |
| a calculus or concretion found in the stomach or intestines of certain animals, esp. ruminants, formerly reputed to be an effective remedy for poison. |
To be “in one's birthday suit” is to be completely naked (as people are at birth).