I was struck by this use of incredulous in an old trailer for the 1932 Universal Studios film The Mummy, starring Boris Karloff:
My first reaction was that it should be incredible – since incredulous means ‘unbelieving’ or ‘disbelieving’, not ‘unbelievable’ – but it seemed unlikely that such a ‘mistake’ would have slipped through unnoticed. So I looked it up.
Sure enough, incredulous used to also mean ‘incredible’, but the OED labels the sense obsolete. Its citations date from 1616 to 1750 (the first is from Twelfth Night: ‘no incredulous or unsafe circumstance’), and there’s an adverbial use from 1533 (‘incredulous quick’). The more familiar incredulous ‘unbelieving’ dates to at least 1578.
Given the apparently short attested life of incredulous ‘incredible’, it’s curious that the usage should show up in mainstream 20thC use – unless it didn’t really disappear in the mid-18thC. Browsing examples of incredulous on Google Books from 1751–1931, I found a single example out of the first 200:
Now the most incredulous thing, as it always appeared to me, is that the casket containing Mr. Lincoln’s remains stayed in that position for two years. (Journal of the Senate of the General Assembly of the State of Iowa, Vol. 42, 1927)
I also came across a letter on the subject to the editor of the Correct English and Current Literary Review, from 1905:
I wish you would kindly inform me whether the sentence, “It seems incredulous to me that you could be so negligent in this respect,” is correct: or, is “incredible” the proper word to use? Please give reasons.
All other examples were of incredulous meaning ‘unbelieving’. Even allowing for repeats, of which there were a few, the proportion is telling.
To answer the letter writer: I would say incredible is the more proper word, because it is fully standard, whereas incredulous in that context was not then in common or reputable usage; its acceptability would have declined in tandem with its use.
The Columbia Guide to Standard American English (1993) says incredulous meaning unbelievable was ‘Standard until about two hundred years ago, but not today, except humorously as a malapropism’. Merriam-Webster’s Dictionary of English Usage concurs, finding that the usage was ‘once in good repute’:
But this sense of incredulous had fallen into disuse by the end of the 18th century. Its reappearance in recent years has been sporadic, although there are signs that the usage may be growing more widespread.
It offers a few modern examples (e.g., from People and ESPN.com) but implies that it is not standard, and recommends that incredulous be restricted to the ‘disbelieving’ sense. Whether the ‘incredible’ use of incredulous in the trailer for The Mummy was anachronistic, or meant as a humorous malapropism, or something else, I can’t say for sure.
Here’s the trailer in full: