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."
"conscious," c. 1600, from Latin conscientem, present participle of conscire "to be conscious" (see conscience). Also as a noun, "a conscious being" (c. 1770).
1540s, "having a conscience;" 1580s, of actions, "consonant with right or duty;" 1640s, of persons, "governed by conscience." It and conscioned "appear to be popular formations from conscion, taken as a singular of conscien-ce" by misapprehension of the "s" sound as a plural inflection [OED]. See conscience. Related: Conscionably. Obsolete from early 18c. but fossilized in its negative, unconscionable.
c. 1600, "knowing, privy to" (poetic), from Latin conscius "knowing, aware," from conscire "be (mutually) aware," from assimilated form of com "with," or "thoroughly" (see con-) + scire "to know" (see science). The Latin word probably is a loan-translation of Greek syneidos.
The sense of "knowing or perceiving within oneself, sensible inwardly, aware" is from 1630s, perhaps a shortening of conscious to oneself (1620s). Also compare the Latin sense evolution in conscience. From 1650s as "aware (of a fact)." Sense of "active and awake, endowed with active mental faculties" is from 1837. Related: Consciously.
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.
It forms all or part of: abscissa; conscience; conscious; ecu; escudo; escutcheon; esquire; nescience; nescient; nice; omniscience; omniscient; plebiscite; prescience; prescient; rescind; rescission; science; scienter; scilicet; sciolist; scission; schism; schist; schizo-; schizophrenia; scudo; sheath; sheathe; sheave (n.) "grooved wheel to receive a cord, pulley;" shed (v.) "cast off;" shin (n.) "fore part of the lower leg;" shingle (n.1) "thin piece of wood;" shit (v.); shive; shiver (n.1) "small piece, splinter, fragment, chip;" shoddy; shyster; skene; ski; skive (v.1) "split or cut into strips, pare off, grind away;" squire.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit chindhi, chinatti "to break, split up;" Avestan a-sista- "unsplit, unharmed," Greek skhizein "to split, cleave, part, separate;" Latin scindere "to cut, rend, tear asunder, split;" Armenian c'tim "to tear, scratch;" Lithuanian skiesti "to separate, divide;" Old Church Slavonic cediti "to strain;" Old English scitan, Old Norse skita "to defecate;" Old English sceað, Old High German sceida "sheath;" Old Irish sceid "to vomit, spit;" Welsh chwydu "to break open."
Old English scitan, from Proto-Germanic *skit- (source also of North Frisian skitj, Dutch schijten, German scheissen), from PIE root *skei- "to cut, split." The notion is of "separation" from the body (compare Latin excrementum, from excernere "to separate," Old English scearn "dung, muck," from scieran "to cut, shear;" see sharn). It is thus a cousin to science and conscience.
"Shit" is not an acronym. The notion that it is a recent word might be partly because it was taboo from c. 1600 and rarely appeared in print (neither Shakespeare nor the KJV has it), and even in "vulgar" publications of the late 18c. it is disguised by dashes. It drew the wrath of censors as late as 1922 ("Ulysses" and "The Enormous Room"), scandalized magazine subscribers in 1957 (a Hemingway story in Atlantic Monthly) and was omitted from some dictionaries as recently as 1970 ("Webster's New World").
Extensive slang usage; meaning "to lie, to tease" is from 1934; that of "to disrespect" is from 1903. Shite, now a jocular or slightly euphemistic and chiefly British variant of the noun, formerly a dialectal variant, reflects the vowel in the Old English verb (compare German scheissen); the modern verb has been influenced by the noun. Shat is a humorous past tense form, not etymological, first recorded 18c. To shit bricks "be very frightened" attested by 1961. The connection between fear and involuntary defecation has generated expressions in English since 14c. (the image also is in Latin), and probably also is behind scared shitless (1936).
Alle þe filþ of his magh ['maw'] salle breste out atte his fondament for drede. ["Cursor Mundi," early 14c.]
c. 1600, "one who studies and resolves cases of conscience," from French casuiste (17c.) or Spanish casuista (the French word also might be from Spanish), Italian casista, all from Latin casus "case" (see case (n.1)) in its Medieval Latin sense "case of conscience." Often since 17c. in a sinister or contemptuous sense "over-subtle reasoner, sophist." Related: Casuistic; casuistical; casuistically.