Pictograph of a human foot with toes - the original meaning was simply 'foot' (the planted, unmoving foot). From a foot stationary on the ground came the abstract sense 'to stop, halt, prevent.'
止 is its own Kangxi radical (#77) and shows up as a meaning-component inside
步,
走,
武,
正 - all motion-related characters. One of the oldest oracle-bone graphs in continuous use.