In one's birthday suit

Origin

The phrase "in one's birthday suit" draws on a simple, universal truth: we are all born naked. This euphemistic and playful idiom emerged to describe a state of undress by comparing it to the 'garment' one wears at birth. While the exact moment of its coinage is debated, its first known appearance in print is often attributed to the Scottish poet Robert Fergusson in his 1773 work, "The Daft-Days," humorously depicting a character in a state of natural vulnerability. It's a gentle, almost whimsical way to refer to nudity, evoking the innocence of infancy rather than the bluntness of the literal word.