Starting in the 1960s, the phrase shoe-in started to be used as a misspelling of shoo-in, and usage has exponentially increased since then. Maybe someday we'll see the former replace the latter; right now, shoo-in is only used three times more than shoe-in, and that gap is decreasing, so that's interesting to observe. Shoo-in actually has a really fascinating history: it traces to rigged horse races in the 1930s, when some paid-off jockeys would drop back to let a chosen horse win. The idea was that the competitions were so obviously fixed that it almost resembled the other contestants shooing the chosen horse toward the finish line. That gradually grew to have less of a cheating connotation and be more associated with things that seem guaranteed in general.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.