The Smurfs are named after a word that meant nothing at all. It filled a gap when its inventor's memory failed him at the table.
The inventor was Peyo, the pen name of Belgian cartoonist Pierre Culliford. Wikipedia retells his account of a meal with fellow cartoonist André Franquin on the Belgian coast. Peyo could not think of the word for salt and asked Franquin, in French, to pass the schtroumpf. Franquin played along: here is the schtroumpf, and when you are done schtroumpfing, schtroumpf it back. The pair spent the rest of the weekend talking in schtroumpf.
Etymonline tells it slightly differently, saying the forgotten word was "salt-shaker." It also hedges with "supposedly," since the story rests on Peyo's own telling.
Peyo turned the joke into characters. The little blue creatures first appeared in La Flûte à six trous ("The Flute with Six Holes"), a story in his Johan and Pirlouit series. Wikipedia dates their first full appearance in the magazine Spirou to October 23, 1958. Their speech kept the original joke, swapping "smurf" for ordinary nouns and verbs.
The English name came by way of the Netherlands. Most languages coined their own names, but Etymonline says the Dutch translation, Smurf, was carried into English. That happened in 1979, when the Wallace Berrie company launched Smurf toys for English-speaking markets. Wikipedia notes an earlier English edition had briefly called them goblins.
The French word has an accidental echo. Wikipedia points out that schtroumpf sounds like German Strumpf, meaning "sock." That is only a resemblance, not a source: the name came from the dinner joke.
Other languages went their own way, which makes the Smurfs a small lesson in translation. German calls them Schlümpfe and Italian Puffi. Spanish uses Pitufos, which Wikipedia says Miguel Agustí coined after seeing a sculpture of the Ox of Patufet in Barcelona. Hebrew speakers call them Dardasim, from an old word for a slipper.