Etymology
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remorse (n.)

"intense and painful self-condemnation and penitence due to consciousness of guilt; the pain of a guilty conscience," late 14c., from Old French remors (Modern French remords) and directly from Medieval Latin remorsum"a biting back or in return," noun use of neuter past participle of Latin remordere "to vex, torment disturb," literally "to bite back, bite again" (but seldom used in the literal sense), from re- "back, again" (see re-) + mordēre "to bite," which is perhaps from an extended form of PIE root *mer- "to rub away, harm."

The sense evolution was via the Medieval Latin phrase remorsus conscientiæ (Chaucer's remors of conscience, also translated into Middle English as ayenbite of inwit). Middle English also had a verb, remord "to strike with remorse, touch with compassion, prick one's conscience" (late 14c.), from Latin remordere. Richard Brome's "Merry Beggars" (1641) delighted that they had 

No bargains or accounts to make,
No land or lease to let or take:
Or if we had, should that remore us
When all the world's our own before us[?]  
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remorseless (adj.)

"unpitying, cruel," 1590s, from remorse + -less. Related: Remorselessly; remorselessness.

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remorseful (adj.)

"characterized by remorse, burdened with a painful sense of guilt and penitence due to consciousness of guilt," 1590s, from remorse + -ful. Related: Remorsefully; remorsefulness.

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*mer- 

Proto-Indo-European root meaning "to rub away, harm." Possibly identical with the root *mer- that means "to die" and forms words referring to death and to beings subject to death.

It forms all or part of: amaranth; ambrosia; amortize; Amritsar; immortal; manticore; marasmus; mare (n.3) "night-goblin, incubus;" morbid; mordacious; mordant; moribund; morsel; mort (n.2) "note sounded on a horn at the death of the quarry;" mortal; mortality; mortar; mortgage; mortify; mortmain; mortuary; murder; murrain; nightmare; post-mortem; remorse.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit mrnati "crushes, bruises," mriyate "to kill," martave "to die," mrta- "died, dead," mrtih "death," martah "mortal man," amrta- "immortal;" Avestan miriia- "to die," miryeite "dies," Old Persian martiya- "man;" Hittite mer- "to disappear, vanish," marnu- "to make disappear;" Armenian meranim "to die;" Greek marainein "to consume, exhaust, put out, quench," marasmus "consumption," emorten "died," brotos "mortal" (hence ambrotos "immortal"); Latin mors (genitive mortis) "death," mori "to die;" Armenian merani- "to die;" Gothic maurþr, Old English morþ "murder;" Old Irish marb, Welsh marw "dead;" Lithuanian mirti "to die," mirtis "death;" Old Church Slavonic mreti "to die," mrutvu "dead;" Russian mertvyj, Serbo-Croatian mrtav "dead."

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compunction (n.)

mid-14c., "remorse, contrition (for wrongdoing, as a means of attaining forgiveness of one;s sins)," from Old French compunction (12c., Modern French componction), from Late Latin compunctionem (nominative compunctio) "remorse; a stinging or pricking" (of the conscience), noun of action from past-participle stem of Latin compungere "to severely prick, sting," from com-, here perhaps an intensive prefix (see com-), + pungere "to prick, pierce" (from suffixed form of PIE root *peuk- "to prick").

The Latin word was used in a figurative sense by early Church writers. Originally a much more intense feeling, similar to "remorse," or "contrition."

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twinge (n.)

1540s, "a pinch, a nipping," from obsolete verb twinge "to pinch, tweak," from Old English twengan "to pinch," from Proto-Germanic *twangjan (source also of Old Frisian thwinga, Old Norse þvinga, Danish tvinge, Dutch dwingen, Old High German thwingan, German zwingen "to compel, force"), from PIE *twengh- "to press in on" (see thong). Meaning "sharp, sudden minor pain" is recorded from c. 1600. Figurative sense (with reference to shame, remorse, etc.) is recorded from 1620s.

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angst (n.)

1944, from a specialized use in psychology of German Angst "neurotic fear, anxiety, guilt, remorse," from Old High German angust, from Proto-Germanic *angustu-(source also of Old Frisian ongost, Old High German angust, Middle Dutch ancst "fear," also Old English enge, Old Saxon engi, Gothic aggwus "narrow"), from PIE *anghosti-, suffixed form of root *angh- "tight, painfully constricted, painful." Compare anger.

George Eliot used it (in German) in 1849, and it was popularized in English early 20c. by translation of Freud's work, but as a foreign word until 1940s. Old English had a cognate word, angsumnes "anxiety," but it died out.

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ruth (n.)

c. 1200, perhaps late Old English, ruthe, "misery, sorrow, grief;" also "pity, compassion, sorrow for the misery of another" (often in have ruth, take ruth); also "remorse, repentance, regret;" from Old Norse hryggð "ruth, sorrow," from hryggr "sorrowful, grieved" (see rue (v.)) + Proto-Germanic abstract noun suffix *-itho (see -th (2)).

Or else formed in English from reuwen "to rue" on the model of true/truth, etc. The Old English word was rue (n.2).

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contrition (n.)

c. 1300, contrycyun, contricioun, "brokenness of spirit for having given offense, deep sorrow for sin or guilt with the purpose of not sinning again," from Old French contriciun "contrition, remorse;

a break, breach" (Modern French contrition) and directly from Late Latin contritionem (nominative contritio) "grief, contrition," noun of action from past-participle stem of conterere, literally "to grind" (see contrite). The modern sense is a figurative use in Christianity. The word was sometimes used in Middle English in the literal Latin sense "a crushing" (mid-14c.).

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inwit (n.)

"inward awareness of right or wrong" (a word formed to translate Latin conscientia), early 13c., "conscience;" c. 1300, "reason, intellect," from in (adj.) + wit (n.).

Þese ben also þy fyve inwyttys: Wyl, Resoun, Mynd, Ymaginacioun, and Thoght [Wycliffe, c. 1380]

Not related to Old English inwit, which meant "deceit." Joyce's use of it in "Ulysses" (1922) echoes the title of the 14c. work "Ayenbite of Inwyt" ("Remorse of Conscience," a translation from French) and is perhaps the best-known example of the modern use of the word as a conscious archaism, but it is not the earliest.

If ... such good old English words as inwit and wanhope should be rehabilitated (and they have been pushing up their heads for thirty years), we should gain a great deal. [Robert Bridges, English poet laureate, 1922]
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