A meaning of 'incredulous' that might leave you incredulous
Some readers of a post from this morning faulted a judge for writing, "The Court finds [Mother]'s testimony incredulous" instead of "… incredible." A Facebook post by one reader asked, "Did I miss when 'incredulous' came to mean 'not believable' instead of 'not believing'?"
I too thought the judge's usage was nonstandard, but it turns out that it's quite well-attested. The Merriam-Webster, for instance, gives "incredible" as one of the definitions of "incredulous," though a Usage Note notes,
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.
And the Oxford English Dictionary echoes this, though noting that this usage is "Obs[olete]":
[2.] a. Not to be believed; = incredible adj. Obs.
a1616 Shakespeare Twelfth Night (1623) iii. iv. 78 No obstacle, no incredulous or vnsafe circumstance….
1750 W. Warburton Julian in Wks. (1811) VIII. 207 The crosses on the garments..must appear a very incredulous circumstance.
My advice is to keep "incredulous" for its more common meaning of "unbelieving"—or, better yet, changing phrasing such as "He was incredulous" to "He was skeptical" or "He didn't believe …"—and to say "incredible" when you mean "incredible." As best I can tell, "incredulous" meaning "incredible" is no longer standard, though it once was. (The American Heritage and the Random House, for instance, don't even list this meaning; see also this Google Ngrams search which shows how much more common "incredible statement" is than "incredulous statement," even counting some incredulous-meaning-unbelieving uses.)
Still, "incredulous" in the sense of "incredible" isn't a pure innovation—rather, it's a (likely inadvertent) revival of a term that was once apparently normal. That doesn't make this usage standard or advisable, but it does show how some nonstandard and inadvisable usages are actually pretty old.