To act in a deranged, reckless, or unconventional manner. Refers to the eponymous character of the novel Don Quixote, who tosses his hat over a windmill (which he imagines is a giant) as a challenge to it. Sarah is always trying to buck social conventions, flinging her bonnet over the windmill whenever possible.I know you like to take risks in business, but don't fling your bonnet over the windmill.