The weapon that made Star Wars feel ancient and futuristic at once has a name built from a 17th-century cavalry word.
Popular Science reports that George Lucas's first draft called the weapons "lazerswords." Live Science says early drafts call them "laser swords." Either way, it was a plain pulp label: a laser shaped like a sword. Wikipedia notes that in early drafts the blades were fairly ordinary plasma weapons. The mystical Force, and with it the Jedi's special skill, came in later revisions.
In the final name, "light" replaced "laser" and "saber" replaced "sword." The Historical Dictionary of Science Fiction dates "lightsaber" to 1975, in Lucas's third-draft script, where Luke mentions "my father's lightsaber." Dictionary.com gives 1977, the year the film came out, and defines it as a Star Wars sword with a blade of laser energy.
The second half of the word has its own long road. Etymonline traces "saber" to French sabre, used in English since the 1670s. The French took it from German Sabel, which probably came from Hungarian szablya, roughly "tool to cut with." Etymonline defines a saber as a heavy, single-edged sword, usually slightly curved.
The prop was as improvised as the name was polished. Wikipedia credits set decorator Roger Christian with building the hilts from old Graflex camera flash handles found in a London photography shop. The glowing blades were added afterward, rotoscoped onto the film by animator Nelson Shin.
The sound came from the same scrap-heap spirit. Sound designer Ben Burtt combined the hum of idling motors in old film projectors with buzz picked up from a television set. He has called the result a "fifty-fifty kind of combination" of the two.
The lightsaber has company in the dictionary. The Historical Dictionary of Science Fiction dates Jedi and Padawan to 1973 and Wookiee to 1974. It finds droid as early as 1952, so not every Star Wars word was new.