Ever been unable to think of a word for what you wanted to say? Strictly speaking, linguistic determinism claims that this is impossible.
Wouldn't that be an "unknown unknown"? My mother-tongue, Romanian, has a word called dor, whose exact meaning/translation I could never find in any English-written text. It doesn't describe unicorns in heavens or any such non-existent thing, in fact there is a similar term called saudade in Portuguese (http://en.wikipedia.org/wiki/Saudade), but the thing is that in order to get its full meaning you need to have it included in a larger text, because words by themselves don't mean anything.