Maybe you're looking for "absent antonyms"? Absent Antonyms
It isn't very multisyllabic though.
Someone asked me if there's a special term for words that are used with the negative prefix only like inept/ungainly, etc. I know unpaired and negative orphan, but I think they were after something else- is there another term for them, something multisyllabic that groans with the weight of its Greek/Latin roots?
Again, not what they're after.