incredulous adj.

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.

"disposed to believe, uncritical with regard to beliefs," 1570s, from Latin credulus "that easily believes, trustful," from credere "to believe" (see credo). Related: Credulously; credulousness.

Alas, my Friends, credulous incredulity is a strange matter. But when a whole Nation is smitten with Suspicion, ... what help is there? Such Nation is already a mere hypochondriac bundle of diseases; as good as changed into glass; atrabiliar, decadent; and will suffer crises. Is not Suspicion itself the one thing to be suspected, as Montaigne feared only fear? [Carlyle, "French Revolution"]

"disbelieving frame of mind," early 15c., incredulite, from Old French incrédulité, from Latin incredulitatem (nominative incredulitas) "unbelief," noun of quality from incredulus "unbelieving" (see incredulous).