Green’s Dictionary of Slang

underpants, occas. a nappy/diaper; occas. sing. as didy.

1933		J. Conroy Disinherited 269: Has to wear a didy like a baby.
1935		J. Conroy World to Win 18: Terry Hurley has made his own way in this hard world sence he was past yellerin’ his didies.
1961		(con. 1945) G. Forbes Goodbye to Some (1963) 150: He says scathingly [...] ‘Let’s go home [...] and get our didies changed’.
1961		L. McMurtry Horseman, Pass By (1997) 14: Go on [...] befo’ you wets you didy.
1986		S. King It (1987) 120: If he screwed up – if the baby shat her didies after the sitter went home and the mess was still in them when Mom got home ...
2021		T. Pluck Boy from County Hell 242: ‘I don’t expect you to remember, you were in your didies’.