What to call words with permanent prefix, but no unprefixed form? (ex: nonchalant, untoward)

Question

What do you call prefixed words with no unprefixed counterpart? For example, there's no such thing as a "chalant person". Bad behavior may be "untoward", good behavior is never "toward". What are these words called and why do they exist? Did unprefixed versions ever exist?

Answer 1

Edwin Ashworth's comment (which is beneath your question) sent me to the linked question, the answer to which in turn sent me to the following site: http://www.2wheels.org.uk/return/absent-antonyms.asp. "2wheels" published there his work in progress regarding missing antonyms, which is the term I believe you are looking for. The list (a work in progress) of missing antonyms suggested by "2wheels" and the comment which precedes it are as follows:

"In no particular order, my list looks like this at the moment, and those with asterisks qualify as perfect absent antonyms, where the presumed positive is, so far as I know, never found. Those without asterisks have positive forms that are occasionally found, but very much more rarely than the negative form:

• "Unrequited (requited is very rarely used).

• Unfathomable (used very much more than fathomable, which, when used, is usually in a negative construction such as scarcely fathomable).

• Inordinate (much more common than ordinate).

• Untrammelled (trammelled is rarely found).

• Untoward (though toward is a common word, untoward still qualifies under rule six).

• *Intact (Tact is a commonly-found word, if not a commonly found quality, but its usual meaning (conveying the sense of polite discretion) is unrelated to intact, so the word qualifies under rule six. Intact, meaning untouched or undamaged, has a presumed positive form tact, meaning damaged, but one rarely hears American soldiers saying: "We blew that city up pretty well - man, it was totally tact."

• *Infinity (finite, the adjective, is common enough, but I have never seen the presumed abstract noun finity, which ought, were it to exist, to convey the concept of boundedness).

• *Inept (the adjective ept is absent).

• *Nonsensical (sensical is not found).

• Unmissable (missable is not recognised by my computer, though it is now perhaps becoming unofficially common when used as a sarcastic comment on, for example, a tedious museum).

• Insufferable (sufferable seems much rarer).

• Uncouth (couth is found, but much less often than its negative counterpart).

• *Unspeakable (speakable is not found).

• Unstinting (stinting is very rarely found).

• Unthinkable (thinkable is very rarely found, and then usually in a negative construction such as barely thinkable).

• *Insomnia (somnia is not found).

• Unperturbed (found more often than the odd-looking perturbed).

• *Indescribably (describably does not make it onto my computer's spell-checker; it might just scrape its way into a large dictionary, one of which to hand I do not have).

• *Ruthless (qualifies as a wild-card under rule 9, having no apparent connection with Ruth.)

• Ineffable (effable is rarely seen or heard)"