Etymology
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birthday suit (n.)

first attested 1730s, but probably much older. The notion is the suit of clothes one was born in, i.e., no clothes at all. Compare Middle English mother naked "naked as the day one was born;" Middle Dutch moeder naect, German mutternackt.

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Definitions of birthday suit

birthday suit (n.)
informal terms for nakedness;
in his birthday suit
Synonyms: raw / altogether
From wordnet.princeton.edu