the.com/runaway

means A person, animal, or thing that has fled or escaped, or something (a hit, a success) that has moved out of control in its own direction — also used of a situation already decided beyond recall.

from Straightforward compound of 'run' + 'away,' stitched together in Middle English as a way to describe someone who literally ran off — a fleeing servant, a bolting horse. By the 1700s it had settled into a noun of its own, and later stretched metaphorically to cover runaway trains, runaway inflation, and runaway hits: anything that's gotten away from whoever was supposed to be holding the reins.