Etymology
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immemorial (adj.)
c. 1600, from French immémorial "old beyond memory" (16c.), from Medieval Latin immemorialis, from assimilated form of in- "not, opposite of" (see in- (1)) + Latin memorialis of or belonging to memory" (see memorial (n.)). Something immemorial is ancient beyond memory; something immemorable is not worth remembering. Latin immemor meant "unmindful, forgetful, heedless."
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immemorable (adj.)
"not memorable," 1550s, from Latin immemorabilis "not worth mentioning; silent," from assimilated form of in- "not" (see in- (1)) + memorabilis (see memorable). In English it occasionally has been used to mean "old beyond memory," but that sense is best left to immemorial.
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