I think it came from old cowboy stories that would have two strand sgoing at once that would join up so...youd have pop out on the prairie having broken his ankle and the horse has bolted, "meanwhile back at the ranch" mom has gone off to tend to a neighbour, strictly adjuring little Timmy not to leave the ranch..she has just got out of sight when Pop's horse comes back riderless....what is little Timmy to do ???