mid-15c., "variability," from Middle French incertitude (14c.), from Late Latin incertitudinem (nominative incertitudo) "uncertainty," from in- "not, opposite of" (see in- (1)) + certitudo "that which is certain," from Latin certus "sure, certain" (see certain). From c. 1600 as "doubt, hesitation." Middle English also had incertain "uncertain" and incertainty "uncertainty," both from Old French, but both have been displaced by forms in un-.
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