"small, smooth stone," c. 1300, pibel, from Old English papolstan "pebblestone," a word of unknown origin. Perhaps imitative. Some sources compare Latin papula "pustule, pimple, swelling." Historically there is no precise definition based on size other than that it is smaller than a cobble. Related: Pebbly. Pebble-dashing "mortar with pebbles incorporated" is by 1941 (pebble-dash in the same sense is by 1902).