1823, "not wholly conscious, feebly conscious" (implied in subconsciously), from sub- + conscious. First attested in De Quincey. The noun, in the psychological sense ("mental processes taking place without consciousness"), is attested from 1886, from adjectival sense "occurring in the mind, but not in consciousness;" earlier noun was subconsciousness (1845).[subconscious etymology, subconscious origin, 英语词源]