unobtainium predates Avatar https://en.wikipedia.org/wiki/Unobtainium

and at least that exposition makes more sense then the "fountain of youth brain juice" in the sequel, when the humans can literally reincarnate themselves without having to cross interstellar space to do it.

Avatar was a famously wrong use of the very old term for something that is (no longer) available anywhere.

Unobtanium was a thing in fiction long before Avatar.