Etymology
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conscientious (adj.)

1610s, of persons, "controlled by conscience, governed by the known rules of right and wrong;" of conduct, etc., "regulated by conscience," 1630s, from French conscientieux (16c.; Modern French consciencieux), from Medieval Latin conscientiosus, from Latin conscientia "sense of right, moral sense" (see conscience). Related: Conscientiously; conscientiousness.

Conscientious objector is from 1896, in reference to those with religious scruples about mandatory vaccination. Military sense predominated from World War I.

After a chequered career full of startling episodes and reversals, the Vaccination Bill becomes virtually the Vaccination Act. In Parliament the hottest of the contest centred round the conscientious objector. [The Lancet, Aug. 13, 1898] 

Slang shortening conchy is attested from 1917.

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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.). 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.

Þese ben also þy fyve inwyttys: Wyl, Resoun, Mynd, Ymaginacioun, and Thoght [Wyclif, c. 1380]
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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conscience (n.)
Origin and meaning of conscience

c. 1200, "faculty of knowing what is right," originally especially to Christian ethics, later "awareness that the acts for which one feels responsible do or do not conform to one's ideal of right," later (late 14c.) more generally, "sense of fairness or justice, moral sense."

This is from Old French conscience "conscience, innermost thoughts, desires, intentions; feelings" (12c.) and directly from Latin conscientia "a joint knowledge of something, a knowing of a thing together with another person; consciousness, knowledge;" particularly, "knowledge within oneself, sense of right and wrong, a moral sense," abstract noun from conscientem (nominative consciens), present participle of conscire "be (mutually) aware; be conscious of wrong," in Late Latin "to know well," from assimilated form of com "with," or "thoroughly" (see con-) + scire "to know," probably originally "to separate one thing from another, to distinguish," related to scindere "to cut, divide," from PIE root *skei- "to cut, split" (source also of Greek skhizein "to split, rend, cleave").

The Latin word is probably a loan-translation of Greek syneidesis, literally "with-knowledge." The sense development is perhaps via "to know along with others" (what is right or wrong) to "to know right or wrong within oneself, know in one's own mind" (conscire sibi). Sometimes it was nativized in Old English/early Middle English as inwit. Russian also uses a loan-translation, so-vest, "conscience," literally "with-knowledge."

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