1759 (in translations of Voltaire), from French optimisme (1737), from Modern Latin optimum, used by Gottfried Leibniz (in "Thodice," 1710) to mean "the greatest good," from Latin optimus "the best" (see optimum). The doctrine holds that the actual world is the "best of all possible worlds," in which the creator accomplishes the most good at the cost of the least evil.
En termes de l'art, il l'appelle la raison du meilleur ou plus savamment encore, et Theologiquement autant que Gomtriquement, le systme de l'Optimum, ou l'Optimisme. [Mmoires de Trvoux, Feb. 1737]
Launched out of philosophical jargon and into currency by Voltaire's satire on it in "Candide." General sense of "belief that good ultimately will prevail in the world" first attested 1841 in Emerson; meaning "tendency to take a hopeful view of things" first recorded 1819 in Shelley.