Memento

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late Middle English (denoting a prayer of commemoration): from Latin, literally ‘remember!’, imperative of meminisse .

wiktionary

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Borrowed from Latin mementō(“remember”), imperative form of meminī(“I remember”).

etymonline

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memento (n.)

late 14c., "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," second person singular imperative of meminisse "to remember, recollect, think of, bear in mind," a reduplicated form, related to mens "mind," from PIE root *men- (1) "to think." Meaning "a hint or suggestion to awaken memory, a reminder, an object serving as a warning" is from 1580s; sense of "keepsake" is recorded by 1768.