World Wide Words observes that this expression is so often wrongly spelled “shoe in” that “…it’s likely it will eventually end up that way.” The term originated with horse racing, where a shoo-in was the certain winner of a rigged race. It comes “…from the verb shoo, meaning to drive a person or animal in a given direction by making noises or gestures, which in turn comes from the noise people often make when they do it.” The shift to the description of rigged horse races began in the early 1900s. C.E. Smith used the term ‘shooed in” to describe winning through manipulation in 1908.