"metal cap on a rod," 1610s, ferule, earlier verrel (early 15c.), from Old French virelle "ferrule, collar" (12c. Modern French virole), from Medieval Latin viriola "bracelet," diminutive of Latin viriae "bracelets," from a Gaulish word akin to Old Irish fiar "bent, crooked," from PIE *wi-ria-, from root *wei- (1) "to turn, twist" (see wire, n.). Spelling influenced by Latin ferrum "iron."