It was originally heavy tent canvas weight cotton impregnated with the same stuff that painters use to prepare canvas for oil paints.
Then it was commercialised and had 'drying agents' added to the mixture of waxes and assorted stuff. Think old fashioned sou'westers without the shiny surface.
Nowadays it's more likely to be just a slightly heavier weight of our wax jacket fabric, but with almost greenland wax.
Somewhere on line there's Filson's recipe for the stuff.
In our damp climate, it's still cold and chilly 'unless' you wear wool underneath.
Cotton's fine as a lining in a cold dry area, but blooming little use in our Atlantic climate.
cheers,
Toddy