means Moving on foot faster than a walk, with a moment where both feet leave the ground at once, or more loosely keeping something operating or in motion.

from From Old English rinnan and the related irnan, meaning to flow or move quickly — the same Germanic root flows through Dutch rennen and German rinnen. The original sense of 'flowing,' like water in a stream, survives in phrases such as 'running water' and 'the river runs,' which is why a tap, a nose, and an engine can all 'run' without taking a single step.