Wiktionary
n. A hat designed to be compressed and carried under the arm, formerly worn on dress occasions by men in the 18th century, and later in the United States Army by general and staff officers.
Usage examples of "chapeau-bras".
In a moment Chauvelin was on his feet and with eyes dilated, lips parted in awed bewilderment, he was gazing towards the open window, where astride upon the sill, one leg inside the room, the other out, and with the moon shining full on his suit of delicate-coloured cloth, his wide caped coat and elegant chapeau-bras, sat the imperturbably Sir Percy.
Fernand had just plied the knocker with his accustomed impatience when a tall man wearing a huge caped coat and chapeau-bras, which further enhanced his stature, accosted him by slapping him lustily on the back.