means Kept in reserve as an extra, beyond what's currently needed, so it's ready to step in if something runs out or breaks; as a verb, to refrain from harming someone or to give up something you could keep.

from From Old English 'sparian,' to refrain from harming or to use frugally, and the related adjective 'spær,' meaning thrifty or scanty. It has Germanic cousins — German 'sparen,' to save, sits in the same family. The thread running through all its senses is restraint: holding something back, whether that's your hand, your wrath, or one extra tire in the boot.