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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Definitions of immemorial

immemorial (adj.)
long past; beyond the limits of memory or tradition or recorded history;
time immemorial
From wordnet.princeton.edu