Slinky is a rare toy name that was found in the dictionary rather than invented.
The toy started as a mistake. In 1943, according to Smithsonian Magazine, the mechanical engineer Richard James was designing a device to keep equipment and shipments secure on rocking Navy ships. He dropped the coiled wires he was working with, and they tumbled end over end across the floor. He saw a possible toy in that motion.
A year later his wife Betty went looking for a name. She scoured the dictionary in 1944 and settled on "slinky," which the Smithsonian glosses as "sleek and sinuous in movement or outline." The word suited a coil that seems to pour itself down a staircase.
The adjective was not new. Etymonline records slinky from 1921, describing women or clothes as sinuous and slender. Before that, a 1895 use meant lank, lean or flaccid. It is built from the verb slink plus the suffix -y. Slink goes back to the Old English slincan, "to creep, crawl," used of reptiles, from a Proto-Germanic root that Etymonline says is related to sling. By the late 14th century it described people or animals moving off quietly.
So a word for creeping snakes became a word for a stylish dress, and then the name of a spring. The 1940s coil did not creep. It tumbled. But the sense of smooth, flowing movement held.
The Jameses founded James Industries in 1945 with a $500 loan, and the Smithsonian says the toy reached shops that year. At a Gimbels department store in Philadelphia that Christmas season, about 400 Slinkys sold out in under two hours at $1 each. Richard James received a patent on January 28, 1947.
Etymonline dates the toy's marketing to 1948, three years after the Smithsonian's date, so the exact year of launch is not settled across sources. The name itself is not in doubt.