'Unbelievable' definitions:

Definition of 'unbelievable'

From: WordNet
adjective
Beyond belief or understanding; "at incredible speed"; "the book's plot is simply incredible" [syn: incredible, unbelievable] [ant: believable, credible]
adjective
Having a probability too low to inspire belief [syn: improbable, unbelievable, unconvincing, unlikely]

Definition of 'Unbelievable'

From: GCIDE

Synonyms of 'unbelievable'

From: Moby Thesaurus