I'm rather curious where the whole ethics-morality distinction came from. It seems to be a rather recent and non-specialist usage.

Indeed, the distinction is not at all clear in language. Most philosophers of ethics I've talked to prefer to 1) use the words interchangeably, or 2) stipulatively define a distinction for the purposes of a particular discussion. In the wild, I've seen people assume distinctions between ethics and morality in both directions; what some call "ethics" others call "morality" and vice-versa.

Comment where I enumerate some common definitions