Wiktionary
gym knickers
n. (context UK English) Large girls' underpants traditionally worn as shorts during school sports activities.
Usage examples of "gym knickers".
By mid-afternoon, a duffel coat with a missing toggle, a pair of tights ripped with such savagery that the legs were entirely separated, and a pair of navy-blue gym knickers were on their way to the county police laboratory, carefully packaged to preserve any forensic traces.
And after a speedy search I found the drawer where she kept all her gym knickers.