Entry 1 sense Webster, 1913 Fleetness · Fleet·ness · IPA /ˈfliːt.nəs/ 01 n. Swiftness; rapidity; velocity; celerity; speed; as, the fleetness of a horse or of time. 1. Swiftness; rapidity; velocity; celerity; speed; as, the fleetness of a horse or of time.