Etymology of Conscience and Conscious
Conscience: Middle English (also in the sense 'inner thoughts or knowledge'): via Old French from Latin conscientia, from conscient- 'being privy to', from the verb conscire, from con- 'with' + scire 'know'
Conscious: late 16th century (in the sense 'being aware of wrongdoing'): from Latin conscius 'knowing with others or in oneself' (from conscire 'be privy to') + -ous
Source: Oxford Dictionaries
1. As Science means Knowledge, Conscience etymologically means Self-knowledge ; and such is the meaning of the word in Latin and French, and of the corresponding word in Greek ; (conscientia, conscience). But the English word implies a Moral Standard of action in the mind, as well as a Consciousness of our own actions. It may be convenient to us to mark this distinction of an internal Moral Standard, as one part of Conscience ; and Self- knowledge, or Consciousness, as another part.
Source: The Elements of Morality, Including Polity, by William Whewell
2. The popular name for the Moral Faculty applies to a cognitive power : Con-science (con-scientia, ), conjoint knowledge. Conscience and Consciousness are not only similarly compounded, but are originally two forms of the same word — conscientia.
Source: Handbook of Moral Philosophy, by Henry Calderwood, pg. 65
3. Conscience, Consciousness. Self - Consciousness. Etymologically considered, these words mean the same thing. Conscientia in Latin corresponds both in formation and meaning to o-wt'iSijo-is in Greek. In the former language it sometimes denotes the joint knowledge of several people, e. g. of those engaged in a conspiracy. When used, however, for an affection of the individual mind, it has for the most part the ethical meaning which is ordinary with us, i.e. it signifies the faculty of moral judgment. Conscience, however, in French and Italian has a larger import, and denotes the whole of what we call consciousness. Indeed, it seems in former days to have done so in English, being that used both by Hooker and Bacon.1 Now, however, we avail ourselves of the two words, the generic term consciousness to denote the whole exercise of the mind's reflex action whereby it both feels and knows, and knows that it feels and knows, and the specific term conscience to denote that particular action of the former whereby it recognises the moral character of everything which we feel, say, or do, which is susceptible of such character.
Source: A Dictionary of English Philosophical Terms, Francis Garden, pg. 33
4. (a) The etymology of `consciousness' and `conscience'
The word `consciousness' has its Latin root in conscio, formed by the coalescence of cum, meaning `with', and scio, meaning `know'. In its original Latin sense, to be conscious of something was to share knowledge of it, with someone else, or with oneself. The knowledge in question was often of something secret or shameful, the source of a bad conscientia, a bad conscience. A `weakened' sense of conscientia coexisted in Latin with the stronger sense which implies shared knowledge: in this weak sense conscientia was, simply, knowledge. All three senses (knowledge shared with another, knowledge shared with oneself and, simply, knowledge) entered the English language with `conscience', the first equivalent of conscientia. The words `conscious' and `consciousness' first appear early in the 17th century, rapidly followed by `self-conscious' and `self-consciousness' (Lewis, 1960).
Source: Brain (2001) 124 (7): 1263-1289. doi: 10.1093/brain/124.7.1263
5. The noun consciousness ... though it was formed later than conscience; it was not formed in order to express a new meaning, but was at first a useless synonym.
Source Studies in Words, C.S. Lewis, Cambridge University Press, 1967, pg. 210
6. CONSCIENCE (conscientia, Gewissen, joint or double knowledge), that power by which we have knowledge of moral law. This word is similarly compounded with " Consciousness." Conscience expresses more abstractly, " Knowledge with ; " Consciousness, the state of the mind as possessing knowledge — knowledge of self and of present experience."
Source: Vocabulary of Philosophy, Psychological, Ethical, Metaphysical; with Quotations and References, by William Fleming and Henry Calderwood. 1887, pg. 82
7. The word 'conscience' does not immediately denote any moral faculty by which we approve or disapprove. Conscience supposes, indeed, the existence of some such faculty, and properly signifies our consciousness of having acted agreeably or contrary to its directions.
Source: The Theory of Moral Sentiments, Adam Smith, pg. 409
8. Tracing the development of consciousness in the early modern period is complicated by the fact that both Latin and French, the two primary languages in which philosophy was written, have a single term that could mean either (a) moral conscience, or (b) consciousness. For Latin, the term is conscientia; in French it is conscience. (The verb and adjective forms are similarly ambiguous.) These terms were used with both of their major senses (along with other minor senses) in the seventeenth century, and so any interpretation of the important texts will have to be sensitive to this potential ambiguity.
But, more importantly, the underlying reason for this ambiguity is due to the shift in meaning that was taking place during the seventeenth century. Conscientia and conscience, both of which primarily signified a moral conscience prior to the seventeenth century, were now taking on a new, purely psychological, meaning. And the philosophers of this period were among the main figures influencing this shift in meaning.
Source: Jorgensen, Larry M., "Seventeenth-Century Theories of Consciousness", The Stanford Encyclopedia of Philosophy (Fall 2010 Edition), Edward N. Zalta (ed.),http://plato.stanford.edu/archives/fall2010/entries/consciousness-17th
9. See also the Oxford English Dictionary.