The bikini is named after a nuclear test site. The name was chosen on purpose, within a week of the first blast.
On July 1, 1946, the United States set off an atomic bomb at Bikini Atoll in the Marshall Islands. It was the first test of Operation Crossroads, and the first American nuclear test in peacetime. The atoll's 167 residents had already been moved to Rongerik Atoll to make way for it.
Four days later, on July 5, a Paris engineer named Louis Réard showed a new swimsuit at the Piscine Molitor, a public pool. He called it the bikini. Réard was competing with the designer Jacques Heim, who had launched a small two-piece called the Atome that May. Réard pitched his as smaller than the world's smallest bathing suit.
No professional model would wear it. Réard hired Micheline Bernardini, an 18-year-old nude dancer from the Casino de Paris, to present it.
Why name a swimsuit after a bomb site? The standard references quote no explanation from Réard himself, but the reading at the time was clear. A 1947 French magazine, quoted by Etymonline, called the word as stinging as the explosion itself. Etymonline treats the name as a likely analogy between the bomb's impact and the garment's.
The atoll's own name is older and gentler. In Marshallese it is Pikinni, usually explained as pik, "surface," plus ni, "coconut": roughly "coconut place." Etymonline calls that explanation uncertain. European charts once called it Eschscholtz Atoll, after a naturalist on Otto von Kotzebue's voyages of 1816 and 1817.
The word crossed into English fast. Merriam-Webster dates its first English use to 1947, and Etymonline to 1948.
Then English speakers misread it. Because bi- means "two" in words like bicycle, people treated the bikini as a "bi-kini," two pieces of something called a kini. The designer Rudi Gernreich's monokini of 1964 ran with the joke, and trikini followed in 1967. Tankini came much later, in 1998. None of these make etymological sense, since the "bi" was never a prefix. It is the first syllable of an island's name.