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.