(Also occasionally in singular) any of various (men's) garments resembling both boxer shorts and briefs; (now) specifically snugly fitting underpants of stretch fabric, with legs typically extending to the mid-thigh.
Origin
1940s; earliest use found in The Charleston Daily Mail.
Definition of boxer briefs in US English:
boxer briefs
noun
US
(Also occasionally in singular) any of various (men's) garments resembling both boxer shorts and briefs; (now) specifically snugly fitting underpants of stretch fabric, with legs typically extending to the mid-thigh.
Origin
1940s; earliest use found in The Charleston Daily Mail.