Green’s Dictionary of Slang

underwear, usu. women’s.

1901		D. Cotsford Society Snapshots 76: Lady Baba. Only allowing her one poor thou. a year for pin money. . .How could she pay her bills on that? Lady Bobo. Hardly enough for undies. Lady Baba. Let alone nighties.
1906		Punch 30 May 384/3: She’d blouses for Sundays, / And marvellous ‘undies’ / Concocted of ribbons and lace.
1918		B.E.F. Times 22 Jan. (2006) 279/2: They’re seven boxes of Penn’s dear delightful undies.
1922		Joyce Ulysses 335: As for undies they were Gerty’s chief care and who that knows the fluttering hopes and fears of sweet seventeen (though Gerty would never see seventeen again) can find it in his heart to blame her?
1928		Advocate (Burnie, Tas.) 20 Mar. 2/4: The little playlet was entitled ‘Grundy v. Undie’ [...] a display to trace the evolution of ‘undies’ through the ages.
1930		Sporting Times 4 Jan. 1/2: If he wrote that a girl was wearing black and white undies you’d know she wasn’t wearing any at all.
1936		Sydney Morn. Herald 12 May 8s/2: Look, Mother! Frivolous undies.
a.1948		(con. early 1930s) C. McKay Harlem Glory (1990) 23: A pair of fancy French undies.
1951		R. Service ‘Rosenstein’ in Lyrics of a Low Brow 87: So each unto his job; and mine / Is making ‘undies’ for the dames.
1963		N. Dunn Up the Junction 109: Nice pair of woollen undies here.
1972		‘Judge Dread’ ‘Big Eight’ 🎵 Little Bo-Peep, loses sheep, she also loses undies, / Loses them five times a day, and always six on Sundays.
1986		T. Winton That Eye, The Sky 110: Her jeans are so tight you can nearly read the size of her undies.
1999		Indep. Rev. 19 July 13: [Compaq ad] ‘Online undies’ takes off big time and weeks later you’ve got designer underpants coming out of your ears.
2004		T. Winton ‘Sand’ in Turning (2005) 167: His undies sagged [...] the way they were the day he had pooped his pants at school.
2015		N. Cummins Tales of the Honey Badger [ebook] On reflection there may have been a small amount in the undies.