unimpressionable 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 impressionable.
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 sensitive or susceptible to impression
“an unimpressionable mind”
Synonyms, antonyms and the broader and narrower relations for unimpressionable come from Princeton University’s WordNet. Full sources and licences.