The phrase "shoo-in" originated in the late 1920's from the sport of horse racing. It referred to the winner of a fixed race. It later fell into common usage to mean "a sure winner" or something that holds no challenge. A "shoo-in" was a horse that could win a race even if it just lazily plodded its way around the track.
The phrase comes from "shoo" (as in "Shoo fly, don't bother me") which is to drive an animal away using verbal commands and/or hand gestures.
"Shoo-in" has found a home in modern politics. Many candidates are considered "shoo-ins" for their positions because they are either running without competition, their district is fixed heavily in favor of their party, or their opponent is so loathsome that very few people would vote for them.
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