"important vein of an ore or mineral in rock," 1849, from mother (n.1) + lode (n.); said to be a translation of Mexican Spanish veta madre, a name given to rich silver veins. The American use is first in reference to a conspicuous vein of quartz rich in gold discovered during the gold rush in the Sierra Nevada of California. The colloquial or figurative sense of "richest source of something" is by 1916.
"low-waisted two-piece women's bathing suit," 1948, from French, coined 1947, named for the U.S. A-bomb test of June 1946 on Bikini, the 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 (compare c. 1900 British slang assassin "an ornamental bow worn on the female breast," so called because it was very "killing," also the figurative sense of bombshell).
Bikini, ce mot cinglant comme l'explosion même ... correspondant au niveau du vêtement de plage à on anéantissement de la surface vêtue; à une minimisation extrême de la pudeur. [Le Monde Illustré, August 1947]
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-.