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.
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."
"conscious," c. 1600, from Latin conscientem, present participle of conscire "to be conscious" (see conscience). Also as a noun, "a conscious being" (c. 1770).