Shoo in..........?

[jon1968]

.......or is it shoe in, meaning a certainty?

Does anyone know where this comes from?

[jno]

shoo. Coming first is as easy as telling a pet to shoo, or something like that.

[Clanad]

The following from Online Etymology Dictionary:
Shoo, v.:

1622, "to drive away by calling 'shoo,' " from the exclamation (1483), instinctive, cf. Ger. schu, It. scioia. Shoo-in "easy winner (especially in politics)" (1939) was originally a horse that wins a race by pre-arrangement (1928; the verb phrase shoo in in this sense is from 1908). Shoo-fly, admonition to a pest, was popularized by a Dan Bryant minstrel song c.1870, which launched it as a catch-phrase that, according to H.L. Mencken, "afflicted the American people for at least two years." Shoo-fly pie is attested from 1935.