I’m sure you’ve had the conversation, at some point in your life, where you’ve discussed the fact that some words have prefixes or suffixes that indicate an antonym, but no antonym exists. For example, we say someone or some act is uncouth, but not couth; our hair can be unkempt, but not kempt; you can be disgruntled, but not gruntled. Those examples all depend on simple prefix removal, but such words need not be constrained in that way. Consider, for example, feckless, which might be matched with a term like feckful (but isn’t). These terms are frequently referred to, unimaginatively, as unpaired words.
A related issue is the cranberry morpheme, one of my favorite terms in linguistics. A cranberry morpheme refers to a part of a word that can only be used in context with another part; that is, it is a morpheme that doesn’t have meaning unless connected to other, specific morphemes. (Think of a morpheme as the smallest unit of language that is significant enough to distinguish meaning; contrast with a phoneme, the smallest unit of sound that can be distinguished in a given language.) Morphemes are classified as either bound or unbound; the former cannot stand alone, while the latter can. All prefixes and suffixes are bound morphemes, but can be attached to a variety of other morphemes.
Cranberry morphemes don’t have a meaning separate from their particular bound morphemes. You know what pre- means even if I don’t type out the rest of a word. Contrast that with the most common cranberry morphemes, such as -ceive in perceive, receive, or conceive. -ceive is not a suffix; it has no meaning outside of the context of the morphemes it attaches to. (Although it seems like there’s some intuitive connection between what the morpheme -ceive is doing in each of those words.) So with cran-: unlike, say, the black in blackberry (an unbound morpheme), cran- has no intrinsic semantic definition.
There’s a quite-old but cool discussion of unpaired words and cranberry morphemes at Language Hat. Incidentally, I didn’t first become interested in cranberry morphemes through their namesake word, but rather due to aimless thinking about the term lukewarm. (Could you have lukehappy feelings?) That’s actually a good reminder that morphemes can be homophonous (sound the same) but have separate meanings: the luke- in lukewarm is unrelated– semantically, syntactically, and etymologically– to the name Luke.