[in.cred.u.lous] adj [L incredulus, fr. in- + credulus credulous] (1579) 1: unwilling to admit or accept what is offered as true: not credulous: skeptical
2: incredible
3: expressing incredulity -- in.cred.u.lous.ly adv usage Sense
2. has been revived in this century after a couple of centuries of disuse. Although it is a sense with good literary precedent--among others Shakespeare used it--many people think it is a result of confusion with incredible, which is still the usual word in this sense.