1894 Brewer’s / Y / Y
A letter resembling “y” was the Anglo-Saxon character for th (hard); hence ye, yt, ys, etc., are sometimes made to stand for the, that, this.
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ABCDEFGHIJKLMNOPQRSTUVWXYZ
Entry taken from Dictionary of Phrase and Fable, edited by the Rev. E. Cobham Brewer, LL.D. and revised in 1895.