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1759 (in translations of Voltaire), from French optimisme (1737), from Modern Latin optimum, used by Gottfried Leibniz (in "Théodicée," 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 Géométriquement, le systême de l'Optimum, ou l'Optimisme. [Mémoires de Trévoux, 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.
What a programmer is full of after fixing the last bug and just before actually discovering the *next* last bug. Fred Brooks's book "The Mythical Man-Month" contains the following paragraph that describes this extremely well.
All programmers are optimists. Perhaps this modern sorcery especially attracts those who believe in happy endings and fairy god-mothers. Perhaps the hundreds of nitty frustrations drive away all but those who habitually focus on the end goal. Perhaps it is merely that computers are young, programmers are younger, and the young are always optimists. But however the selection process works, the result is indisputable: "This time it will surely run," or "I just found the last bug.".
See also Lubarsky's Law of Cybernetic Entomology.