The word “ethical” builds on the word “ethos” that comes from the Greek “ήθος”.
It is the English equivalent adjective to the Greek “ηθικός”. It means to be related to “moral principles” or, more straightforward, to “be morally good“.
This second meaning is precisely the original meaning of the word as it comes from the Greek language. In fact, if one was to trace the equivalent words among the languages, one would find that the Greek “ήθος“, “ethos” in the English language, is the word “mōrēs” in Latin.
Seeing how the word “moral” in the English language roots from the Latin “mōrēs”, it makes sense that “ethical” (Greek “ήθος”) is “moral (Latin “mōrēs”)“.
The opposite of “ethical” is “unethical”, which means “immoral”.