(in your) birthday suit
noun phrase (humorous)

- not wearing any clothes

The Cambridge Dictionary

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Birthday suit is a humorous expression for the state of being naked. It literally means to wear what one wore on their day of birth, that is, nothing.

The phrase serves as a euphemism for writers and speakers seeking a delicate or amusing way to refer to the state of nudity without reference to intimate regions.

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ORIGIN

At least 200 years old, birthday suit was recorded as early as 1771, but was probably in use as a spoken expression before that. The phrase was widely used in the 20th century and remains popular even to this day.

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SYNONYMS

Au naturel, adamite, disrobed, in the altogether, in the buff, full-frontal, stark naked, in the nude, in the raw, unclad, unclothed, without a stitch, in the state of nature

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Practice OWAD in an English conversation, say something like:

“In the summertime in Munich, don’t be surprised to see people in the Englischer Garten sunbathing in their BIRTHDAY SUITS.”

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Paul Smith