improbable has 6 distinct synonyms spread across 3 of its 3 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 probable.
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.
having a probability too low to inspire belief
too improbable to admit of belief
“a tall story”
Synonyms, antonyms and the broader and narrower relations for improbable come from Princeton University’s WordNet. Full sources and licences.