Old English spura, spora (related to spurnan "to kick," see spurn), from Proto-Germanic *spuron (cf. Old Norse spori, Middle Dutch spore, Dutch spoor, Old High German sporo, German Sporn "spur"), from PIE *spere- "ankle" (see spurn).
Generalized sense of "anything that urges on, stimulus," is from late 14c. Meaning "a ridge projecting off a mountain mass" is recorded from 1650s. "Widely extended senses ... are characteristic of a horsey race." [Weekley] Expression on the spur of the moment (1801) preserves archaic phrase on the spur "in great haste" (1520s). To win one's spurs is to gain knighthood by some valorous act, gilded spurs being the distinctive mark of a knight.