Definition: unclothed skin; nakedness
Some people like to collect autoantonyms, words which mean a thing and its opposite, such as splice and cleave. No one ever seems to include birthday suit in these lists, even though the word has been used to mean both “a suit worn on one’s birthday” and “a state of nudity” (possibly because the clothing sense is now mostly obsolete). The use of birthday suit to indicate a state of nudity has been common since at least the middle of the 18th century.
The obliging Bride, to remove all incumbrances, stripp’d to her Buff, and two Women held a sheet between her and the Clergyman, while he performed his Office; but she having forgot her Cap at undressing, in the midst of the Ceremony it came into her Mind, and she pull’d that off too, and flung it on the Bed, and was married to her Spouse (if not in a Wedding Suit) in her Birth Day Suit.
—The Pennsylvania Gazette (Philadelphia, PA) 18 Jun. 1752