Scuba is one of the few everyday words that started life as a technical label.

Both Etymonline and Merriam-Webster say it is an acronym for "self-contained underwater breathing apparatus." Etymonline dates it to 1952 in American English. Merriam-Webster gives 1952 as the first known use. Etymonline adds that "scuba-diving" is attested by 1956.

The man usually credited with the name is Christian Lambertsen. He invented a free-swimming breathing device for the Office of Strategic Services, the wartime American intelligence agency. The date is unsettled. A U.S. Army article says his career began in 1941 with the invention, while Wikipedia says he invented the rebreather in 1939. ARSOF History says only that he designed a prototype before the United States entered the war. The National Academies memorial tribute says he was a 23-year-old medical student when he presented it.

At first the device had other names. The National Academies tribute says it was called the Lambertsen Lung and eventually the Lambertsen Amphibious Respiratory Unit, or LARU. The Army article says he later renamed it the Self-Contained Underwater Breathing Apparatus.

The date of that renaming is 1952 in two sources. The National Academies tribute says Lambertsen and a colleague wrote a paper that year for the National Academy of Sciences, which "resulted in the acronym SCUBA." Wikipedia says he coined the acronym in 1952, when he patented the modified apparatus. ARSOF History, run by U.S. Army Special Operations, says he coined it after the war. The tribute does not say outright that he coined the term, and Etymonline and Merriam-Webster name no coiner, so the credit rests on those two sources.

The word then did what successful acronyms do. People stopped capitalizing it, and it became a plain noun. Wikipedia compares it to "radar." Merriam-Webster now defines it as a breathing device with its own portable supply of compressed gas, and notes it is often used before another noun, as in "scuba diver."

Wikipedia adds that Jacques-Yves Cousteau and Emile Gagnan's open-circuit regulator carried the name "Aqua-Lung," which is a trademark. A generic term was therefore useful, and scuba filled the gap.