E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Bonnet.
A pretended player at a gaming-table, or bidder at an auction, to lure others to play; so called because he blinds the eyes of his dupes, just as if he had struck their bonnet over their eyes.
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“A man who sits at a gaming table, and appears to be playing against the table; when a stranger appears the Bonnet generally wins.”—The Times.
