Longjohns is a synonym of underpants.
As nouns the difference between longjohns and underpants
is that longjohns is (long johns) while underpants is underwear, clothing covering the genitalia.
underpants
English
Synonyms
* (underwear) boxers, briefs, bloomers, (UK'') pants, panties, (slang) undies, tighty whities, knickers, drawers, (''slang, New Zealand ) gruds, longjohns, thong, underwear
See also
(Other terms of interest)
* boxer shorts
* kegs
* knickers
* panties
* Y-fronts