unobtanium is engineering humour. Its un-obtainable-ium and linked together to sound like an element.
It means that the material properties you are looking for are impossible to get. So you tell who ever is wanting you to design a component that to be able to meet there specs it needs to be made out of unobtanium. When they ask for a costing of course the unobtanium costs a stupid amount of money which would mean the design would be uneconomic.