budgie smugglers

Etymology
A jocular reference to a man's tight-fitting swimming costume or swimsuit appearing as if he has a budgerigar concealed inside it, i.e. his bulging genitals.

Noun

 * 1)  A style of tightfitting men's swimming costume cut like underwear briefs that covers the buttocks and groin but not the legs and reveals the bulge of the genitals; especially worn in surf lifesaving and in swimming races.