immemorial
adj.
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.
〔李〕[im-(in-);memor;-iala.] a.无法追忆的； 远古的 ←memor,member (L memor)=mindful 记住的
〔蒋〕[im-不，无，memorial记忆的;’无法记忆的”] 无法追忆的，太古的，远古的