I love orphaned words. (Not sure what the technical term for this is, chemo brain can’t pull it up.) English is full of them, once you start looking around. Two that leap to mind are “condign” and “ulterior”.
Not much besides punishment can be condign. Ever heard of a condign ham sandwich? Likewise, one can have an ulterior motive, but rarely sees an ulterior diagnosis. There are some limited uses for both words, and of course one can always ironically force a word for stylistic reasons, but mostly these words appear in one and only one place.
What are your favorite examples of this?
Also, bonus word: “preantepenultimate”.