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c.1400, "Psalm cxxxi in the Canon of the Mass" (which begins with the Latin word Memento and in which the dead are commemorated), from Latin memento "remember," imperative of meminisse "to remember, recollect, think of, bear in mind," a reduplicated form, related to mens "mind" (see mind (n.)). Meaning "reminder, object serving as a warning" is from 1580s; sense of "keepsake" is first recorded 1768.
"reminder of death," 1590s, Latin, literally "remember that you must die."