Unobtanium
Since the late 1950s, aerospace engineers have used the term
"unobtainium" when referring to unusual or costly materials, or when theoretically considering a material perfect for their needs in all respects, except that it does not exist. By the 1990s, the term was in wide use, even in formal engineering papers such as "Towards unobtainium [new composite materials for space applications]." The word
unobtainium may well have been coined in the aerospace industry to refer to materials capable of withstanding the extreme temperatures expected in reentry Aerospace engineers are frequently tempted to design aircraft which require parts with strength or resilience beyond that of currently available materials.
Later,
unobtainium became an engineering term for practical materials that really exist, but are difficult to get. For example, during the development of the SR-71 Blackbird spy plane, Lockheed engineers at the Skunk Works" under Clarence "Kelly" Johnson used
unobtainium as a dysphemism for
titanium. Titanium allowed a higher strength-to-weight ratio at the high temperatures the Blackbird would reach, but the Soviet Union controlled its supply and was trying to deprive the US armed forces of this valuable resource.[ Eventually, through a European front company, a large quantity of titanium found its way to the United States.
In the 1970's, bicycle magazines, such as
Bike World, sometimes referred to exotic lightweight bicycle parts as being made of unobtanium, although they were commercially obtainable but expensive.
lol That was the single most baffling decision on the whole film. Stood out like dog's balls, didn't it? Enough to yank you out of the film. 
