bikini |biˈkēnē| noun ( pl. -nis ) a very brief two-piece swimsuit for women. • (also bikinis) scanty underpants. ORIGIN 1940s: named after Bikini , where an atomic bomb was exploded in 1946 (because of the supposed [explosive] effect created by the garment). Bikini |biˈkēnē| an atoll in the Marshall Islands, in the western Pacific Ocean, used by the U.S. 1946–58 as a site for testing nuclear weapons.