Immoral means breaking accepted rules of right and wrong. Not illegal necessarily — wrong.
The distinction from illegal is the useful one, since a great many immoral things are perfectly legal, and pointing that out is most of what the word is for. It is worth keeping apart from amoral, which means having no moral sense at all rather than acting against it, so a thief is immoral and a shark is amoral. Immoral earnings is a legal term of art. The noun is immorality.
1adjectiveimmoralContrary to accepted principles of right and wrong.
“He thought the whole arrangement immoral.”
Latin
mōrālis
Old French
moral
Middle English
moral
English
moral
English, today
immoral
Built from
From im- + moral. moral comes from Middle English moral, from Old French moral, from Latin mōrālis. From Middle English moral, from Old French moral, from Latin mōrālis (“relating to manners or morals”) (first used by Cicero, to translate Ancient Greek ἠθικός (“moral”)), from mos (“manner, custom”).
Etymology from Wiktionary, CC BY-SA