libertaire (anarchist) formed from libert (freedom), from Latin libertas and the suffix -aire, from Latin -arius.

The word first appeared in English in 1789 in William Belsham's Essays. This was contrasted with necessitarian, in the context of free will, and was not used in the current sense.

The French word is first attested in a letter in May 1857 by Frenchanarcho-communist Joseph Djacque to anarchist philosopher Pierre-Joseph Proudhon, reading:[1]

In translation:

hence the sense is of extreme left-wing.

The French term was popularized as a euphemism for anarchist in the 1890s, following the lois sclrates, when anarchist publications were banned by law in France.

The sense of pro-property individualist developed in the US in the 1940s, and was popularized in the 1950s. In the 1940s, Leonard Read began calling himself libertarian to contrast with classical liberal.[2] In 1955, Dean Russell also promoted use of the word, writing: Let those of us who love liberty trademark and reserve for our own use the good and honorable word libertarian.[3]

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libertarian - Wiktionary