I noticed a USA Today article today that said "Mary Barra has been a growing force within General Motors. While she wasn't necessarily a shoe-in to be named to the CEO job...". I was pretty sure shoe-in should be shoo-in and m-w.com confirmed that for me, but didn't give any etymology.
In other sources, I have seen references to horse racing and an argument for shoe-in as a salesman would try to get his shoe in the door to further the sale.
What is the etymology?
According to the OED, the adjective (and noun) shoo-in comes from the the collocation of the verb shoo and the preposition/adverb in.
‘Shoo’ in this case has the basic sense of being urged on or in a certain direction (as in “She shooed him away”), and ‘in’ is obviously in the sense of ‘getting in’, i.e., reaching the goal; but the sense is idiomatically “to allow a racehorse to win easily”, rather than simply to urge someone towards goal (vel sim).
The earliest quote they have for this usage is 1908:
There were many times presumably that ‘Tod’ would win through such manipulations, being ‘shooed in’, as it were. [G.E. Smith, Racing Maxims & Methods of ‘Pittsburgh Phil’ ix. 123]
The noun is first attested in 1928, referring to the same race-horsing situation:
A ‘skate’ is a horse having no class whatever, and rarely wins only in case of a ‘fluke’ or ‘shoo in’. [National Turf Digest (Baltimore), Dec. 929/2]
– while the first attestation of the extended political meaning is from 1939:
Bear cagers appear shoo-in for southern division title. [News (San Francisco), 30 Jan. 15/5]
It has no mentions whatsoever of the salesman argument.
I would consider it likely that the racehorse etymology is the true origin; but since racehorse terminology is not in common parlance (anymore, at least), folk etymologies are bound to appear—and an obvious parallel if the only phrase you’ve ever heard spoken is that of getting your shoe in the door. This does strike me as a much more likely ex post facto folk etymology than the racehorse version, since it is based on an expression that is still commonly used in the vernacular.
The OED defines the noun shoo-in as:
In Horse Racing, a predetermined or ‘fixed’ race, or the winner of it. Hence loosely, a horse which is a certain winner.
Their first example of the noun "shoo-in" is in a 1928 horse-racing magazine explaining:
A ‘skate’ is a horse having no class whatever, and rarely wins only in case of a ‘fluke’ or ‘shoo in’.
They say it's from the transitive verb, to shoo in:
With in, to allow a racehorse to win easily. U.S. slang.
Their first use from a 1908 horse-racing book:
There were many times presumably that ‘Tod’ would win through such manipulations, being ‘shooed in’, as it were.
The verb shoo is used for driving animals such as livestock, and there's a clearer link to the earlier meaning of the noun as a fixed race: a chosen horse wins easily as they only had to shoo it in. Later it gained today's meaning of a dead cert.
I found earlier examples of both the noun and the verb.
The verb was used in "Pointers from the Paddock" in The Washington Times, June 23, 1894:
Backers of Syracuse were hot under the collar.
Unkind people said Mattie Chun was "shooed" in.
The noun can be found in The San Francisco Call of August 22, 1895 on "The Bay District Races" with "the favorites all beaten":
Little Pete was a member of the delegation that were in on the "shoo" in the third race. He had several commissioners placing his coin on Model. If the race was to have been a "shoo-in" for Model, there must have been one or two owners that were not in on the deal.
This has both the usual noun "shoo-in" as well as a less common in-less noun "shoo".
The morning Times of September 22, 1895, under "JERSEY WAS "SHOOED" IN":
One of these fixed races was run out of the chute in the fifth event yesterday that was so thoroughly rotten that the talent threw up their hands in horror. Jersey came out and beat the 2-to-5 shot, Forest, with apparent ease, and the one comment that was made on the race was "shoo."
(Here's also verbs and nouns from 1899 and 1903.)
A noun antedating in a 26 June 1910 New-York Tribune details "Vernacular of the Race Track" and helpfully tells us the etymology:
Most of the "regulars" are deeply suspicious of all steeplechase races of late years, and, whenever the favorite falls at one of the obstacles and a long priced leaper wins the race, they loudly call the race a "shoo-in" (a fixed affair, that is, in which the steeplechase racers have arranged to drop to the rear of the "meant" jumper and "shoo" him to the wire, they previously, of course, having got their money down on the horse thus generously treated).
Horse-racing has given rise to quite a lot of modern-day sayings. The aforementioned 1910 Tribune says:
Already we have transferred scores of these baseball and racing terms to the currency of our every day speech, and this sort of transplantation goes on unceasingly.
In searching for answers for EL&U I've discovered quite a few with a horse-racing origin: in good nick, pull it off, and less surprising, racing cert, and from dog hunting and racing: cut corners, get a bye.
