A chapeau bras, or sometimes chapeau-de-bras, was a fashionable hat common in England from about 1760–1840. It was a bicorn or flattish tricorn hat that could be easily flattened so that a gentleman might carry it easily under his arm. Chapeau de bras is in fact French for 'arm hat'.
The early chapeaux-bras were most often tricorns, but later bicorns became popular; the most commonly seen chapeau bras is a crescent worn with the points facing front and back, or alternatively, of course, carried under the arm. These were stylish hats worn by well-dressed gentlemen, and were of no particular use other than show. They were most often made of felt with a leather sweatband, but could also be made of fur, covered in silk, and were adorned by various amounts of piping and folderol. By the 1820s these came to be known as opera hats, although this role was quickly usurped when Antoine Gibus designed the collapsible top-hat in 1840, becoming the new, preferred opera hat.
Bicorns are often associated with the military, especially the navy, and chapeaux-bras join this club; many bicorns of various styles are chapeaux-bras, and most chapeaux-bras are bicorns. The defining feature, however, is that a chapeau bras is designed to be easily flattened.