Chapeau-bras is usually a bicorne but one that will collapse flat.
They were worn as part of 18th Century ceremonial, diplomatic, or naval dress, currently thinking of Admiral Nelson and Napolean Bonaparte.
It basic starting point is a wide brimmed hat with two opposing parts of the brim tied together.
This bicorne (bicorn works as a spelling as well) is at the cheap end of the spectrum, they get much more elaborate with feathers, emblems and gold and silver lace trim, often referred to as cocked hat once they get plumage or fancier.
This was purchased at the Tewkesbury Medieval Battle Fair in 2013 according to the photo below
I also have none collapsing bicorne which I wear more often.
I have found a photo of someone else wearing it points front to back (fore and aft) in the photo below rather than side to side (athwart or port to starboard) as in photos of me above.
