unprofessional 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 professional.
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 characteristic of or befitting a profession or one engaged in a profession
“described in unprofessional language so that high school students could understand it”
Synonyms, antonyms and the broader and narrower relations for unprofessional come from Princeton University’s WordNet. Full sources and licences.