"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]
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."
"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[?]
Old English æppel "apple; any kind of fruit; fruit in general," from Proto-Germanic *ap(a)laz (source also of Old Saxon, Old Frisian, Dutch appel, Old Norse eple, Old High German apful, German Apfel), from PIE *ab(e)l- "apple" (source also of Gaulish avallo "fruit;" Old Irish ubull, Lithuanian obuolys, Old Church Slavonic jabloko "apple"), but the exact relation and original sense of these is uncertain (compare melon).
A roted eppel amang þe holen, makeþ rotie þe yzounde. ["Ayenbite of Inwit," 1340]
In Middle English and as late as 17c., it was a generic term for all fruit other than berries but including nuts (such as Old English fingeræppla "dates," literally "finger-apples;" Middle English appel of paradis "banana," c. 1400). Hence its grafting onto the unnamed "fruit of the forbidden tree" in Genesis. Cucumbers, in one Old English work, are eorþæppla, literally "earth-apples" (compare French pomme de terre "potato," literally "earth-apple;" see also melon). French pomme is from Latin pomum "apple; fruit" (see Pomona).
As far as the forbidden fruit is concerned, again, the Quran does not mention it explicitly, but according to traditional commentaries it was not an apple, as believed by Christians and Jews, but wheat. [Seyyed Hossein Nasr, "The Heart of Islam: Enduring Values for Humanity," 2002]
Apple of Discord (c. 1400) was thrown into the wedding of Thetis and Peleus by Eris (goddess of chaos and discord), who had not been invited, and inscribed kallisti "To the Prettiest One." Paris, elected to choose which goddess should have it, gave it to Aphrodite, offending Hera and Athene, with consequences of the Trojan War, etc.
Apple of one's eye (Old English), symbol of what is most cherished, was the pupil, supposed to be a globular solid body. Apple-polisher "one who curries favor" first attested 1928 in student slang. The image in the phrase upset the apple cart "spoil the undertaking" is attested from 1788. Road-apple "horse dropping" is from 1942.