Etymology of Ethic
Ethics or ethic, according to the Merriam-Webster online dictionary, is a number of rules of behavior that are grounded on concepts of morality (good and bad).
Some etymological dictionaries give a different description of the words ethics and ethic—the first is often defined as the science of morals (from the Greek hē ēthikē tekhnē) while the latter is often defined as the study of morals. Regardless, ethics is the plural form of ethic (Middle English: ethik).
The word ethic dates back to the late fourteenth century (approximately).
It means study of morals.
Its etymological progression goes something like this: first, from the Greek ēthos (a word still used today, e.g. Ethos, Pathos, Logos); secondly, to Latin, as ethice (but see below all the possible variations of ethice in Latin).
Old French changed ethice into ethique.
Middle English then imported the Old French word and transformed it into ethik, which was then, finally, changed into ethic.
Ethics in Latin can be spelled in a variety of ways.
As a noun: ethice, ethices, ethica, ethicae, ethicum, ethici.
As an adjective: ethicus, ethica, ethicum.
de Vaan, Michiel, Etymological Dictionary of Latin and the other Italic Languages, Alexander Lubotsky ed., Leiden: Brill, 2008.
McSparran, Frances, chief editor, The Middle English Compendium, University of Michigan, 2006.
The Oxford English Dictionary, 2nd ed., Clarendon Press, 1989.