TL;DR: An unpaired word is a word that doesn’t exist in common use without a particular prefix or a suffix, or has no natural opposite.
In my new book, Once Upon a Word, I have several pages of charts showing word-building elements: roots, prefixes, and suffixes. As you likely know, you can add prefixes and suffixes to English roots (which often have Latin and Greek origins) in order to build new words.
But there are some words, called unpaired words, that behave a bit strangely because their prefixes and suffixes imply that they have an existing base word or an opposite word—but they don’t.
An unpaired word is a base word (or opposite word) that doesn’t exist (or is very rarely used) without a particular prefix or suffix.
For example, we say we are overwhelmed or underwhelmed, but it’s rare to hear someone say they’re just whelmed. Someone can be reckless, but what does it mean to have reck? You can debunk a rumor, but was the rumor bunked in the first place? Something ridiculous can be nonsensical, but we don’t usually say that things that make sense are sensical.
Here are a few examples of interesting unpaired words that imply the existence of a base word or opposite, and what those base words would mean if they did exist.
Base word: bunk, an outdated American English word. Short for bunkum, meaning “nonsense,” a misspelling of the North Carolina county Buncombe. In 1820, a politician from North Carolina gave a long, boring, nonsensical speech to the US Congress that he addressed to his home county. In most cases, the prefix de- means “down” or “away from,” but in this case it’s what we call an intensifier—it makes the base word stronger. So to add it to bunk is to make it not just nonsense, but complete nonsense.
Base word: gust, from gouster, Middle French for “to taste,” originally from the Latin gustare, “to taste.” So “disgust” means the same as “distaste.”
Base word: nocent, from nocentem, Latin for “harming.” So innocent literally means “harmless.”
Base word: reck, an outdated English word meaning “care” or “consideration,” originally from Old English reccan, “to take care of” or “to be interested in.” So to be reckless is to behave without consideration for safety or consequences.
Base word: scathe, from Old Norse skaða, meaning “to hurt” or “to injure.” So someone unscathed is unhurt.
Base word: whelm, from Old English hwielfan, originally meaning “cover over,” “overthrow,” or “submerge completely” (as in ships in rough waters). So “whelm” once meant the same as “overwhelm,” and “over” was simply a repetitive intensifier.
Learn more in Once Upon a Word: An Etymology Dictionary for Kids (Rockridge Press 2020).