Princeton's WordNet
incredibly, improbably, implausibly, unbelievablyadverb
not easy to believe
Synonyms:
fantastically, improbably, incredibly, fabulously, implausibly, unbelievably
Wiktionary
improbablyadverb
Antonyms:
probablyimprobablyadverb
In an improbable manner; without probability.Antonyms:
probably
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