Merriam-Webster Collegiate Dictionary - incredulous

 adjective  Etymology: Latin incredulus, from in- + credulus credulous  Date: 1579  1. unwilling to admit or accept what is offered as true ; not credulous ; skeptical  2. incredible 1  3. expressing incredulity an ~ stare  • ~ly adverb Usage:  Sense 2 was revived in the 20th 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.