unethical has no recorded synonym. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is ethical.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
not conforming to approved standards of social or professional behavior
“unethical business practices”
Synonyms, antonyms and the broader and narrower relations for unethical come from Princeton University’s WordNet. Full sources and licences.