bonnet (n.)

early 15c., "kind of cap or bonnet worn by men and women," from Old French bonet, short for chapel de bonet, from bonet (12c., Modern French bonnet) "kind of cloth used as a headdress," from Medieval Latin bonitum, bonetum "material for hats," perhaps a shortening of Late Latin abonnis "a kind of cap" (7c.), which is perhaps from a Germanic source.

As a form of head-covering worn by women out-of-doors, late 15c. As a type of mechanical covering device, 1862.

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Definitions of bonnet from WordNet
bonnet (n.)
a hat tied under the chin;
Synonyms: poke bonnet
bonnet (n.)
protective covering consisting of a metal part that covers the engine;
Synonyms: hood / cowl / cowling
bonnet (v.)
dress in a bonnet;