As a keen fan of Terry Pratchett and Jasper Fforde, I'm fascinated with words that logically should exist, but don't. A subset of these are referred to as "lost positives", such as "ebriated" referring to someone who is sober. However, there are other types of theoretical words, such as ones where you apply a prefix which isn't normally used, such as "postcarious", which logically means the unfortunate consequence of something being left in a precarious state.
Is there a word/phrase which is used for these words which only exist in theoretical word-space (preferably in British English)?