I remember the fictional metal unobtainium used in several science fiction movies / stories, often to the point where my friends and I laugh when it is used.
I know it is used in the movie "The Core" as material for the ship, and I more recently remember it being the sought-after material in "Avatar", but is there a history of this fictional metal used elsewhere? Which sci-fi work was this material first used?
Well, Tony Meyer mentions the wiki article in the comments to the question.. but the Citations actually make better reading than the main article:
1.^ "unobtainium, n. A substance having the exact high test properties required for a piece of hardware or other item of use, but not obtainable either because it theoretically cannot exist or because technology is insufficiently advanced to produce it. Humorous or ironical." Listed in "Interim Glossary, Aero-Space Terms," as compiled by Woodford Heflin and published in February, 1958, by the Air University of the US Air Force.
2.^ Since at least the 1950s: Hansen, James R. (1987) "Engineer in Charge: A History of the Langley Aeronautical Laboratory, 1917–1958." The NASA History Series, sp-4305. Chapter 12, recounting an October 1957 meeting, mentions the problems caused by "the lack of a superior high-temperature material (which the Langley structures people dubbed 'unobtainium')" This paragraph in turn cites Becker, John V. "The Development of Winged Reentry Vehicles, 1952–1963," unpublished, dated 23 May 1983.
The first use of the word "unobtainium" that I find is in the 1930 work "Dechema-Monographien - Volume 39 - Page 12" Reference
...Gewicht gleich Null. Ein Amerikaner hat diesen Werkstoff scherzhafterweise einmal das Element "Unobtainium" genannt.
Google Translation
...Weight equal to zero. An American has jokingly called this material once the element "Unobtainium".
The short story "The Skylark of Space (Amazing Stories 1928) uses a "miracle substance" that is later referred to as "unobtainium" reference but this only uses the concept, I am extremely doubtful that this is the first use of the concept.
The Wikipedia article on Unobtainium (as of this writing) has history going back to the 1950s in use by aerospace engineers. Presumably this use was proceeded by a Science Fiction usage, that brought the term to popularity.
I have found some references suggesting that the 1945 work "Animal Farm" was the first use in English of the word 'Unobtainium", but I have been unable to confirm this.
It's a common engineering term, actually. I think it dates back to at least the 50's, though I don't have a reference for that.
It is used a LOT (TV Tropes link) even in real life. According to the link, aluminium was initially so difficult to produce that it was considered unobtanium.
The online Oxford English Dictionary's earliest citation for the word unobtainium (also spelled unobtanium) is from 1956, and is not from a work of fiction. Those without access to the OED can find this information at the Historical Dictionary of Science Fiction:
1956 Marshall (Michigan) Evening Chronicle 27 Feb. 6/8
Scientists are now working on a new metal to be used in making the noses on intercontinental ballistics missiles. The metal is so hard to come by that the scientists have devised a lugubriously-humorous name for it. They call it ‘unobtainium’.
The earliest science-fictional example listed at the HDSF is from David Brin's Startide Rising, mentioned in user140305's answer. Other answers posted here mention the fictional works Animal Farm and The Skylark of Space, which are older than Startide Rising but unfortunately do not use the term "unobtainium".
Unobtainium is found in the core of a planet in the book, Startide Rising, by David Brin (1983). It is being used to construct some sort of super weapon by the ET bad guys.
None of the other moons in the Kthsemenee system had the one attribute this one possessed: a core of almost one percent unobtainium. Already thirty of the Brothers' ships had landed, to begin construction of the Weapon.
This is in answer to the OP's question "Which sci-fi work was this material first used?"
