Etymology
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unbelievable (adj.)

1540s, from un- (1) "not" + believable. Related: Unbelievably.

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Definitions of unbelievable

unbelievable (adj.)
having a probability too low to inspire belief;
Synonyms: improbable / unconvincing / unlikely
unbelievable (adj.)
beyond belief or understanding;
Synonyms: incredible
From wordnet.princeton.edu