"a written note of something to be remembered; a record of something for future reference or consideration," by 1889, shortening of memorandum (q.v.).
mid-15c., "(something) to be remembered," a note of something to be remembered for future reference or consideration, from Latin memorandum "(thing) to be remembered," neuter singular of memorandus "worthy of remembrance, noteworthy," gerundive of memorare "to call to mind," from memor "mindful of" (from PIE root *(s)mer- (1) "to remember").
Memorandum , A terme used when wee write any thing we would not forget. [Cockeram, English Dictionarie, 1623]
"Used originally as mere Latin, and usually abbreviated mem ., to introduce a note of a thing to be done" [Century Dictionary]; by 1540s it came to mean the note itself. The Latin plural is memoranda . Compare also agenda.
Proto-Indo-European root meaning "to remember."
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit smarati "remembers;" Avestan mimar a "mindful;" Greek merimna "care, thought," mermeros "causing anxiety, mischievous, baneful;" Latin memoria "memory, remembrance, faculty of remembering," memor "mindful, remembering;" Serbo-Croatian mariti "to care for;" Welsh marth "sadness, anxiety;" Old Norse Mimir , name of the giant who guards the Well of Wisdom; Old English gemimor "known," murnan "to mourn, remember sorrowfully;" Dutch mijmeren "to ponder."