bonnet (n.) Look up bonnet at Dictionary.com
late 14c., Scottish bonat "soft, brimless hat for men," 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.