The word "spare" can be used as an adjective, noun, or verb. As an adjective, it means something that is extra, not currently needed, or kept for future use. As a noun, it refers to something that is kept as a backup or alternative. As a verb, it means to provide something extra or to refrain from harming or using something.
The word "spare" comes from the Old English "sperian," which means to refrain from using or to save. Over time, it evolved through Middle English into its current form, retaining the sense of being unused or kept in reserve.