1789, "one who holds the doctrine of free will" (especially in extreme forms; opposed to necessitarian), from liberty (q.v.) on model of unitarian, etc. Political sense of "person advocating the greatest possible liberty in thought and conduct" is from 1878. As an adjective by 1882. U.S. Libertarian Party founded in Colorado, 1971. Related: Libertarianism (1849 in religion, 1901 in politics).