curb / kərb/ • n. 1. a stone or concrete edging to a street or path.2. a check or restraint on something.3. (also curb bit) a type of bit that is widely used in western riding. In English riding it is usually only used with a snaffle as part of a double bridle.4. a swelling on the back of a horse's hock, caused by spraining a ligament.• v. [tr.] 1. restrain or keep in check. ∎  restrain (a horse) by means of a curb.2. lead (a dog being walked) near the curb to urinate or defecate.

curb 1 chain or strap passing under a horse's lower jaw and fastened to the bit. XV. Early forms co(u)rbe, prob. f. †co(u)rbe vb. bend, curve (XIV) — (O)F. courber :- L. curvāre CURVE.
Hence curb vb. put a curb on (a horse); restrain, check. XVI.

curb 2 enclosing framework or border. XVI. f. CURB 1. See also KERB.