Is there a term for what 'sheveled' is to 'disheveled'?

This reminded me of Justice Scalia's telling off of a lawyer for using the word choate, which doesn't exist. According to the New York Times article, it's called back-formation:

Stripping the in- from inchoate is known as back-formation, the same process that has given us words like peeve (from peevish), surveil (from surveillance) and enthuse (from enthusiasm). There’s a long linguistic tradition of removing parts of words that look like prefixes and suffixes to come up with “roots” that weren’t there to begin with. Some back-formations work better than others.

In this particular example, it appears the "disheveled" was never an compound of "dis-" and a root word. According to at least one source it was borrowed whole from French and adapted, explaining the lack of "sheveled" in English. So "disheveled" isn't actually an example of the sort of word you're looking to name.

An okay (but not excellent) example would be a word like "rehabilitate". It is a combination of the prefix "re-" and a root word, and "habilitate" isn't a word that enjoys much use today.

Unfortunately, after all that I can't give you a single-word term for such words. In linguistics they're merely called "unproductive roots", but that's just a descriptive phrase for them rather than a dedicated technical term.

I have heard words like this referred to as "cran- morphemes", in that they are false etymons derived by decoupling a valid morpheme from the rest of a valid word, leaving a non-meaningful string with presumed meaning and associationg to the original word. This is by analogy to "cranberry", in which the "-berry" part is a recognizable morpheme that can be appended to meaningful free morphemes in some cases (e.g. "blue-"), but the "cran-" is not.