The story
From unobtain(able) + -ium (suffix typical of names of atomic elements), modelled after uranium and titanium. Compare also the origin of dysprosium.
Etymology from Wiktionary, CC BY-SA 4.0.
unobtainium
un·ob·tain·i·umToday it means: A fictional material or component which, if it existed, would enable one to easily solve a hard problem. Full definition →