in-cred-u-lous / ĭn-krĕj-ə-ləs
1. skeptical; not believing or reluctant to believe
The proof that this is the genuine spot where Noah was buried can only be doubted by uncommonly incredulous people.
From “The Innocents Abroad” by Mark Twain, 1835 – 1910
2. displaying or indicating disbelief
‘Try to forget them!’ he rejoined, raising his haggard face to hers, and regarding her with an incredulous stare.
From “The Old Curiosity Shop” by Charles Dickens, 1812 – 1870
From the Latin adjective incredulus, incredula, incredulum (not believing), a combination of the Latin negative prefix in- (not-, un-) and the Latin adjective credulus, credula, credulum ( easily believing), derived from the root of the Latin verb credo, credere, credidi, creditum (believe, trust, loan).
Thank you to Allen Ward for providing this etymology.