credulity (n.)

early 15c., "faith, belief," from Old French credulité (12c.), from Latin credulitatem (nominative credulitas) "easiness of belief, rash confidence," noun of quality from credulus "that easily believes, trustful," from credere "to believe" (see credo). Meaning "a weak or ignorant disregard of the importance of evidence, a disposition too ready to believe," especially absurd or impossible things, is from 1540s.