Green’s Dictionary of Slang

an absolute certainty.

1931		Dly News (NY) 14 Jan. 42/5: Our Guide says Fletch is a shoe-in to dump Norval.
1941		St Joseph News-Press (MO) 5 May 6/8: Our Boots is what Kentucky Derby fans would call a shoe in.
1950		Barre Dly Times (VT) 18 Mar. 2/2: There isn’t an observer around who isn’t figuring Somerville as a shoe-in for [the] title.
1960		Corpus Christi Caller-Times (TX) 3 Jan. 18/2: [R]unning for reelection now and hoping for a shoe-in a governor then.
1972		Hearings: Cttee Public Welfare 25: Megi was a shoe - in - even counting the illegal absentee ballots - he could not be defeated.
1996		(con. 1964-65) B. Thorpe Sex and Thugs and Rock ’n’ Roll 33: It was not going to be the shoe-in that I thought.