BONNET, originally a soft cap or covering for the head, the common term in English till the end of the 17th century; this sense survives in Scotland, especially as applied to the cap known as a "glengarry." The "bonnet" of a ship's sail now means an addi tional piece laced on to the bottom, but it seems to have formerly meant a piece laced to the top, the term "to vail the bonnet" being found at the beginning of the 16th century to mean "strike sail" (cf. Fr. avaler), to let down. "Bonnet" came to be used of a type of head-covering for women fitting closely to the head; hence, the term is also applied to certain protective devices, as in a steam-engine or safety-lamp, or in slang use to a gambler's ac complice, a decoy. A common use is of the covering which protects from the weather the engine of a motor-car.