BONNET (from Lat. bonetum, a kind of stuff, then the cap made of this stuff), 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 additional 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" (from the Fr. avaler), to let down. In modern times "bonnet" has come to be used of a type of head-covering for women, differentiated from "hat" by fitting closely to the head and often having no brim, but varying considerably in shape according to the period and fashion. The term, by a natural extension, is also applied to certain protective devices, as in a steam-engine or safety-lamp, or in slang use to a gambler's accomplice, a decoy.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)