improbably adv. + Add to list US //ˌɪmˈpɹɑbəbɫi// UK //ɪmpɹˈɒbəbli// im·prob·a·bly SynonymsAntonymsEtymology Synonyms implausiblyincrediblyunbelievably Antonyms believablycrediblyOrigin From improbable + -ly.