(UK) Large girls' underpants traditionally worn as shorts during school sports activities.
1989, Roald Dahl, Matilda:
"But instead of going there, I sneaked into the Trunchbull's room. And after a speedy search I found the drawer where she kept all her gym knickers."
1993, Angela Lambert, The Independent, 11 Oct 1993:
I never knew why they singled me out, though at the time I believed it was because my mother made me wear green gym knickers and everyone else's were navy or white.
2010, Gail Emms, The Guardian, 5 Dec 2010:
I remember being forced to do cross-country runs in horrible navy gym knickers, which is a nightmare when you're not exactly built like Paula Radcliffe [...].