Said to be derived from St. Kilian, whose image was stamped on the “shillings” of Würzburg. Of course this etymology is of no value. (Anglo-Saxon, scylling or scilling, a shilling.)
According to Skeat, from the verb scylan (to divide). The coin was originally made with a deeply-indented cross, and could easily be divided into halves or quarters.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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