While answering the question How often do native speakers use the word “to scathe”? Is it OK if I use it instead of “to injure”?, I described "scathing" and "unscathed" as "fossils", because while they clearly originated as participles of the verb "to scathe", they are used in modern English only as plain adjectives.
However, all the definitions of "fossil word" I can find talk about words with more narrow application in a particular set phrase or idiom, so I'm not sure this is the right term.
Is there a better term for words like this, which have lost their original "siblings" in another part of speech?
