Etymology
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memorandum (n.)

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").

"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.

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memo (n.)

"a written note of something to be remembered; a record of something for future reference or consideration," by 1889, shortening of memorandum (q.v.).

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*(s)mer- (1)

Proto-Indo-European root meaning "to remember." 

It forms all or part of: commemorate; commemoration; mourn; memo; memoir; memorable; memorandum; memorial; memorious; memorize; memory; remember.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit smarati "remembers;" Avestan mimara "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."

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chit (n.1)

"a short letter, note," 1776, short for chitty (1690s), from Mahrati (Hindi) chitthi "letter, note, memorandum," from Sanskrit chitra-s "distinctively marked" (see cheetah).

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IOU 
also I.O.U., I O U, 1610s, originally written IOV (see V); a punning on "I Owe You." "A memorandum or acknowledgement of debt less formal than a promissory note, because no direct promise to pay is expressed." [Century Dictionary]
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note (v.)

c. 1200, noten, "observe, take mental note of, mark carefully," from Old French noter "indicate, designate; take note of, write down," from Latin notare "to mark, note, make a note," from nota "mark, sign, note, character, letter" (see note (n.)). Sense of "mention separately or specially among others" is from late 14c. Meaning "to set down in writing, make a memorandum of" is from early 14c. Related: Noted; noting.

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jurat (n.)
also jurate, "one who has taken an oath," early 15c. (mid-14c. in Anglo-French), from Medieval Latin iuratus "sworn man," noun use of past participle of Latin iurare "to swear" (see jury (n.)). Meaning "official memorandum at the end of an affidavit" (showing when and before whom it was sworn) is from 1796, from Latin iuratum, noun use of the neuter past participle.
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commonplace (n.)

1540s, "a statement generally accepted," a literal translation of Latin locus communis, itself a translation of Greek koinos topos "general topic," in logic, "general theme applicable to many particular cases." See common (adj.) + place (n.). Meaning "memorandum of something that is likely to be again referred to, striking or notable passage" is from 1560s; hence commonplace-book (1570s) in which such were written down. Meaning "well-known, customary, or obvious remark; statement regularly made on certain occasions" is from 1550s. The adjectival sense of "having nothing original" dates from c. 1600.

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query (n.)

1530s, quaere "a question," from Latin quaere "to ask, inquire," "much used as a marginal note or memorandum to indicate a question or doubt, and hence taken as a noun" [Century Dictionary], second person singular imperative of quaerere "to seek, look for; strive, endeavor, strive to gain; ask, require, demand;" figuratively "seek mentally, seek to learn, make inquiry," probably ultimately from PIE root *kwo-, stem of relative and interrogative pronouns. Spelling Englished or altered c. 1600 by influence of inquiry. Compare quest.

Query stands for a question asked without force, a point about which one would like to be informed : the word is used with all degrees of weakness down to the mere expression of a doubt; as, I raised a query as to the strength of the bridge. [Century Dictionary]
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memoir (n.)

early 15c., "written record," from Anglo-French memorie "note, memorandum, something written to be kept in mind" (early 15c., Old French memoire), from Latin memoria (from PIE root *(s)mer- (1) "to remember"). The more specific sense of "a notice or essay relating to something within the writer's own memory or knowledge" is from 17c. Meaning "person's written account of his or her life" is from 1670s. Related: Memoirist.

Biography, Memoir. When there is a difference between these words, it may be that memoir indicates a less complete or minute account of a person's life, or it may be that the person himself records his own recollections of the past, especially as connected with his own life; in the latter case memoir should be in the plural. [Century Dictionary]
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