"low-waisted two-piece women's bathing suit," 1948, from French coinage, 1947, named for U.S. A-bomb test of June 1946 on Bikini, Marshall Islands atoll, locally Pikinni and said to derive from pik "surface" and ni "coconut," but this is uncertain. Various explanations for the swimsuit name have been suggested, none convincingly, the best being an analogy of the explosive force of the bomb and the impact of the bathing suit style on men's libidos (cf. c.1900 British slang assassin "an ornamental bow worn on the female breast," so called because it was very "killing").
As a style of scanty briefs, from 1960. Variant trikini (1967), with separate bra cups held on by Velcro, falsely presumes a compound in bi-.