Some English words exist only as what sound like negative forms, like “incognito.” You never hear about anyone going around “cognito,” after all. But there are also some that were originally positive words, then gained a negative form, then the positive form faded out of use leaving us with only…for example…“ineffable.”
Something that’s “ineffable” can’t be described in language. The word is still used pretty much like John Bulwer used it in his 1650 book Anthropometamorphosis: “Setting forth his ineffable wisdome.” “Ineffable” used to be paired with its opposite, which was of course “effable” — something that can be expressed. “Effable” had a slightly different connotation though; in addition to something that could be described, it also meant something that could be pronounced. This is how John Wilkins used it in 1668: How this Universal Character may be made effable in a distinct Language.
For some reason, “ineffable” is in current use, but “effable” hasn’t been seen or used for more than a century. It could be because it’s more common to need to explain that you can’t describe something than that you can — after all, if you can describe something, you’ll most likely just go ahead and do it. If you need to explain, you might just use the word “describable,” or perhaps “expressable.” But you probably wouldn’t use “enarrable”, even though it means the same thing.
“Enarrable” disappeared from use a couple of centuries before the same thing happened to “effable.” The most recent citation available for “enarrable” is from 1623: “Which may bee Declared, Narrable, Enarrable,” and that’s from Henry Cockeram’s English Dictionarie; or, An Interpreter of Hard English Words.
Cockeram might have seen the need to include “enarrable” as a “hard English word” because “enarrable,” beginning as it does with “en-“, was sometimes mistaken for its opposite, “inenarrable.” For instance, in 1495 William Caxton wrote, in Vitas Patrum: “This daye haue I seen thynges enarrable.” That’s a fairly silly thing to write until you realize that he must have meant “inenarrable,” which means exactly the same as “ineffable” — he saw things he couldn’t describe. Since he was writing it down, my guess would be that he did what most writers do — went on to describe them anyway. Nobody ever seemed to take ineffable or inenarrable seriously.
The story of “inenarrable” is similar to “ineffable” — the positive versions of the words disappeared, but the negatives survived. At least they survived longer; “inenarrable” is now as obsolete as “enarrable.” “Enarrable,” by the way, came from the Latin word “enarrare” (to describe) that’s also the root of “narrate” and “narrative.”
The only times “inenarrable” has appeared in the past hundred years or so is when writers attempt, in the words of Michael Quinion, to “empurple their prose.” The most recent example may be from The Saint in Maimi by Leslie Charteris in 1941 (in context, by the way, this is meant to be funny): “A long draught of the corrosive nectar, to be savoured with the inenarrable contentment which the divine fruit of such a pilgrimage deserved, washed gratifyingly around Mr Uniatz’s atrophied taste buds, flowed past his tonsils like Elysian vitriol, and swilled into his stomach with the comforting tang of boiling acid. He liked it.”
