I seem to recall from my youth, when my vocabulary was many times what it is today, learning a word (started with an 'a' I think) whose meaning was "an old, unused, or archaic word" or something roughly to that effect. I believe it was an adjective though it might have been a noun. This would be quite fun as an autological term. Does anyone recall such a word, or am I deceiving myself?
There are these two ‘a’ words which have a suitable meaning but which are not themselves strictly self-descriptive:
archaism, noun : An archaic word or expression.
anachronism, noun : Anything done or existing out of date; hence, anything which was proper to a former age, but is, or, if it existed, would be, out of harmony with the present
If you’re partial to Greek, then depending on what you’re aiming for, you might consider any of:
- palaeonym (old word)
- archaeonym (ancient word)
- cryptonym (hidden word)
Or the more cryptic:
- spanonym (rare word)
- lanthonym or xechasmonym (forgotten word)
- chamaeonym (a lost word, one that’s on the ground)
If you want most people to be able to guess what you mean, use one from the first set; if you don’t, then use one from the second.
If those won’t do, you’ll have to seek out some of those words’ more recherché poecilonyms.
There is also the term obsolete meaning no longer used. Oxford's COD has obsolete as disused, discarded, antiquated.
Walter W. Skeat uses the term obsolete in his work on etymology, A Concise Etymological Dictionary of the English Language.
Ancient is used quite often, but at least talks about it being old.