The correct spelling is shoo-in, usually with a hyphen. It has been known in that spelling and with the meaning of a certain winner from the 1930s. It came from horse racing, where a shoo-in was the winner of a rigged race.
In turn that seems to have come from the verb shoo, meaning to drive a person or an animal in a given direction by making noises or gestures
These day's the term is associated with short odd's favourites in any sport
Shoo means, "go away, I hate you, you don't belong here, why are you still here, you're not supposed to be here, go away!". so instead of saying all that, you just say shoo!
