|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).