Spur

Spur \spur\ , noun;
1. A u-shaped device that slips over and straps to the heel of a boot and has a blunt, pointed, or roweled projection at the back for use by a mounted rider to urge a horse forward
2. Anything that goads, impels, or urges, as to action, speed or achievement
3. Something that projects and resembles or suggests a gaff; sharp projection

The origin of spur is Old English spura/spora, which derives from Proto-Germanic *spuron from Proto-Indo-European *spere- ("ankle"). Spura/spora is related to Old English spurnan, ("to kick away, reject, scorn, despise") which also ultimately derives from *spere-. The verb version of spur evolved in the 1200's. The generalize sense of "anything that urges on, stimulus" dates to the late 14th century. A now archaic phrase on the spur is first attested in the 1520's and meant "in great haste." This phrase survives in Modern English as spur of the moment.