Per the internet, so grain of salt and all, unobtanium predates Avatar by some time, typically used as a brainstorming device. You know how a physics problem might say “assume a frictionless environment” or something of that nature, in order to focus on a specific point or phenomena? Unobtanium is sort of like that. Picture a bunch of aerospace engineers in the mid-50s, talking about how they’re gonna put a person in space. They’re throwing all their spaghetti against the wall, hoping some of it will stick. One guy stands up and says, according to his calculations, if they can get the mass of the launch vehicle down to X, he’s confident they can do the thing. Unfortunately, material science being what it is at the time, there is nothing that would be light, strong, cheap, and workable enough to fashion such a vehicle, but the math all checks out. These engineers jokingly start referring to the hypothetical material that would satisfy all their needs as “unobtanium”, while they search for practical solutions.

Fast forward 60 years, and Cameron is writing his Pocahontas in Space movie. He needs a name for his MacGuffin, but, being a MacGuffin, it’s entirely irrelevant to the plot outside of the fact that the characters are destined to fight over it. So, he decides to call it unobtanium, since that’s pre-existing shorthand for “rare material that does everything you need it to”, and that’s literally all this material needs to be for the plot.

It’s still silly, sure, but no more or less silly than mechs fighting giant blue people that fuck via ponytail sounding.