That which cannot be obtained; theoretical but not yet real.
It may have come from the movie "The Core," when Delroy Lindo is describing the ship needed to go to the Earth's core, which can only theoretically be built. When someone asks him what it would be made of, he said, "The real name has 37 syllables, but I just call it... unobtanium."
used to describe the material that something you cant find is made out of, like hard to find car parts.
those taillights must be made of unobtanium, i cant find them anywhere