unreasonably has 1 distinct synonym spread across 1 of its 2 senses. 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 reasonably.
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 in a reasonable or intelligent manner
“she reacted unreasonably when she learned she had failed”
to a degree that exceeds the bounds or reason or moderation
“his prices are unreasonably high”
Synonyms, antonyms and the broader and narrower relations for unreasonably come from Princeton University’s WordNet. Full sources and licences.