immemorial (adj.)youdaoicibaDictYouDict[immemorial 词源字典]
c. 1600, from French immémorial (16c.) "old beyond memory," from Medieval Latin immemorialis, from assimilated form of in- "not, opposite of" (see in- (1)) + memorialis (see memorial). Something immemorial is ancient beyond memory; something immemorable is not memorable.[immemorial etymology, immemorial origin, 英语词源]