I always understood “shoo-in” as a corruption of “sure-in,” something or someone who is for sure going to make it or succeed in some bid, such as to win a race or an election. I thought “shoo,” to drive something away, was a completely different expression.
But now, prompted by @zenele, who is usually pretty confident when he speaks of words, I consulted a printed copy of the OED (not trusting any abbreviated online version of it).* It seems that the “sure” explanation is a false etymology based on supposition and that horse-racing is indeed the source of this derivative of “shoo.”
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*And especially not something that turns out to be “Online Etymology Dictionary,” with no clear and visible tie to the OED (Oxford English Dictionary) or Oxford and copyrighted by an individual.
@rebbel, when you consider how many online instances take their authority from other online instances, you really can’t tell very much from such numbers.