ethical: [17] The underlying meaning of Greek ēthos was ‘personal disposition’. It came ultimately from prehistoric Indo-European *swedh-, a compound formed from the reflexive pronoun *swe- ‘oneself’ and dhē- ‘put’ (from which English gets do). Gradually the meaning broadened out to ‘trait, character’ and then ‘custom’, or in the plural ‘manners’ or ‘morals’.
English acquired it, in the sense ‘distinctive characteristic’ (based on the usage of Aristotle), in the 19th century. The Greek derived adjective ēthikós entered English, via Latin ēthicus, as ethic in the 16th century. This had largely been replaced by ethical by the end of the 17th century, but it has survived as a noun (as in ‘the work ethic’), which actually predates the adjective in English by about two hundred years.
The plural usage ethics ‘science of morals’ dates from the beginning of the 17th century. => do
