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.
“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
Braid Bonnet. The old Scotch cap, made of milled woolen, without seam or lining. Glengarry Bonnet.
The Highland bonnet, which rises to a point in front.
He has a green bonnet.
Has failed in trade. In France it used to be customary, even in the seventeenth century, for bankrupts to wear a green bonnet (cloth cap).
He has a bee in his bonnet.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894