Etymology
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loss (n.)
Old English los "ruin, destruction," from Proto-Germanic *lausa- (from PIE root *leu- "to loosen, divide, cut apart"), with an etymological sense of "dissolution." But this seems scarcely to have survived in Middle English, and the modern word, with a weaker sense, "failure to hold, keep, or preserve what was in one's possession; failure to gain or win," probably evolved 14c. from lost, the past participle of lose.

Phrase at a loss "confused, uncertain" (1590s) is a phrase from hunting, in reference to hounds losing the scent. To cut (one's) losses is from 1885, originally in finance. The retailer's loss-leader "advertised product sold at cost or below" (to entice customers in to buy other things as well) is from 1922.
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memory (n.)

late 13c., "recollection (of someone or something); remembrance, awareness or consciousness (of someone or something)," also "fame, renown, reputation;" from Anglo-French memorie (Old French memoire, 11c., "mind, memory, remembrance; memorial, record") and directly from Latin memoria "memory, remembrance, faculty of remembering," abstract noun from memor "mindful, remembering," from PIE root *(s)mer- (1) "to remember."

Sense of "commemoration" (of someone or something) is from c. 1300. Meaning "faculty of remembering; the mental capacity of retaining unconscious traces of conscious impressions or states, and of recalling these to consciousness in relation to the past," is late 14c. in English. Meaning "length of time included in the consciousness or observation of an individual" is from 1520s. 

I am grown old and my memory is not as active as it used to be. When I was younger I could remember anything, whether it had happened or not; but my faculties are decaying now and soon I shall be so I cannot remember any but the things that never happened. It is sad to go to pieces like this, but we all have to do it. ["Mark Twain," "Autobiography"]

Meaning "that which is remembered; anything fixed in or recalled to the mind" is by 1817, though the correctness of this use was disputed in 19c. The word was extended, with more or less of figurativeness, in 19c. to analogous physical processes. Computer sense, "device which stores information," is from 1946. Related: Memories.

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amnemonic (adj.)
"characterized by loss of memory," 1868; see a- (3) + mnemonic.
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amnestic (adj.)
"causing loss of memory," 1861, from Greek amnestia "forgetfulness" (see amnesia) + -ic.
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amnesia (n.)
"loss of memory," 1786 (as a Greek word in English from 1670s), Modern Latin, coined from Greek amnesia "forgetfulness," from a- "not" (see a- (3)) + mnesi- "remembering" (found only in compounds), from stem of mnasthai "to recall, remember," related to mnemnon "mindful," mneme "memory;" from PIE root *men- (1) "to think." The usual word in Greek was amnestia, but this had a specialized sense of "forgetfulness of wrong" (see amnesty).
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mnesic (adj.)

"pertaining to memory," 1898, from Greek mnesikos "of memory," from mnesis "memory" (see mnemonic).

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mnemonic (adj.)

1753, "aiding the memory, intended to assist the memory;" 1825, "pertaining to the memory," a back-formation from mnemonics, or from a Latinized form of Greek mnēmonikos "of or pertaining to memory," from mnēmōn (genitive mnēmonos) "remembering, mindful," from mnēmē "memory, a remembrance, record, an epitaph; memory as a mental faculty," from base of mnasthai "remember," from PIE root *men- (1) "to think." The noun meaning "mnemonic device" is from 1858. Related: Mnemonical (1660s).

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blackout (n.)
also black-out, 1908 in the theatrical sense of a darkened stage, from black (v.) + out (adv.). Figurative sense of "loss of memory" is 1934 (verb and noun); as "a dousing of lights as an air raid precaution," it is recorded from 1935. Verbal phrase black out, in reference to printed or written matter deemed objectionable and covered in black ink, is attested from 1888.
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mnestic (adj.)

"pertaining to memory," 1914, from Greek mnestis "remembrance," related to mnesis "memory" (see mnemonic) + -ic.

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