noun
undershorts; boxer shorts; briefs
Refers to men's underwear, especially loose-fitting undershorts like boxers. The term is somewhat dated; younger speakers may prefer トランクス or ボクサーパンツ.
祖父はいつも猿股をはいている。
My grandfather always wears undershorts.
Compound of 猿 (saru, 'monkey') and 股 (mata, 'crotch'). The origin is uncertain, but it may refer to the way the garment resembles a monkey's crotch or the way it allows freedom of movement like a monkey.