Green’s Dictionary of Slang

(orig. US) long woollen winter underwear, combinations; thus attrib. longjohn.

1941		Sheboygan Press (WI) 16 Oct. n.p.: We have had but three days of rain in the nine weeks we have been here. Last Friday it turned a little cool so we were issued our winter clothes. We all hope we don’t get our ‘long Johns’ for a while because it is too warm yet.
1944		P. Kendall Dict. Service Sl. n.p.: woolen underwear [...] Long Johns.
1948		S.L. Elliott Rusty Bugles I iii: I got to wash three pairs of shorts and all me sox, me two long-uns and a half dozen shirts.
1953		S. Bellow Augie March (1996) 83: His Death was still the old one shriveled mummy longjohns.
1961		M. Braly Felony Tank (1962) 21: A tiny bald man wore longjohns.
1973		S. Ace Stand On It (1979) 16: I tugged down my fireproof Nomex longjohn underwear.
1982		N. Keesing Lily on the Dustbin 36: Both sexes wear ‘long johns,’ ‘coms’ or ‘combies’ (combinations).
1990		R. Campbell Sweet La-La Land (1999) 39: I’d like to see that. I’d like to see Sissie in long johns.
2001		S. King Dreamcatcher 112: There, in a tangle, were the bottoms of McCarthy’s longjohns.
2023		R. Milward Man-Eating Typewriter 254: novak wears long johnathans, insulated lally-covers, woolly mammoth manto [...] .