bare skin; nakedness: They were sunbathing in their birthday suits.
Origin of birthday suit
First recorded in 1745–55
Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for birthday suit
informal, jocular a state of total nakedness, as at birth
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
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Word Origin and History for birthday suit
first attested 1730s, but probably much older. The notion is the suit of clothes one was born in, i.e., no clothes at all. Cf. Middle English mother naked "naked as the day one was born;" Middle Dutch moeder naect, German mutternackt.
Online Etymology Dictionary, © 2010 Douglas Harper
To be “in one's birthday suit” is to be completely naked (as people are at birth).
The New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Idioms and Phrases with birthday suit
Nakedness, as in The doorbell rang, and here I was in my birthday suit. In 18th-century Britain this term originally referred to the clothes one wore on the king's birthday. Later it was jocularly transferred to bare skin, alluding to the condition of a newborn baby.
The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.