The word was first attested in English in 1789 in William Belsham’s Essays: see the quotation. This was contrasted with necessitarian, in the context of free will, and was not used in the more frequently encountered modern sense.
Compare French libertaire (“person with extreme left-wing beliefs, anarchist”), from liberté (“freedom”) + -aire (suffix forming nouns). Libertaire is derived from Latin libertas. The French word was first attested in a May 1857 letter by the French anarcho-communist Joseph Déjacque (1821–1865) to the anarchist philosopher Pierre-Joseph Proudhon (1809–1865), reading:[2] “Anarchiste juste-milieu, libéral et non LIBERTAIRE ”. It was popularized as a euphemism for anarchiste in the 1890s, following the French lois scélérates (literally “villainous laws”) under which anarchist publications were banned.
Sense 3.2 (“believer in right-libertarianism”) developed in the United States in the 1940s and was popularized in the 1950s. In the 1940s, Leonard Read (1898–1983), the founder of the Foundation for Economic Education, a free-market think tank, began calling himself “libertarian” in contrast with a “classical liberal”.[3] In 1955, Dean Russell also promoted the use of the word, writing: “Let those of us who love liberty trade-mark and reserve for our own use the good and honorable word ‘libertarian’.”[4]