Word used to describe any extra unit of hardware, method of transportation, object or intangible concept that cannot reach its maximum potential, mainly as a result of the original being stolen or fiddled with. Occasionally, the spare might have been the one that was stolen by mistake. Often used in a context to convey disbelief that the use of the spare was suggested in the first place.
Guy 1: "Hey, let me use that computer."
Guy 2: "Why don't you just use the computer that you stole...?"
Guy 1: "How can I use that computer when it is a spare?! I need full access!"