There's no space here to go into etymology, but the balmacaan is a walking coat. Despite that, there is little pedestrian about it. It is knee-length, with a Prussian collar, and a full raglan sleeve — i.e. one of those ones with a seam running perpendicular down upper sleeve: the traditional way, and, in this case, the best.
It follows in a great lineage of walking coats, stretching back nearly a century of more. Few outerwear styles, indeed, are so quintessentially British. The balmacaan breaks with this tradition in many ways, however — most obviously in its three-panel body; but in less noticeable other aspects, too.
Still, despite deviating from the norm here and there, what you have here is a walking coat that exemplifies best practice in the form. The collar construction, the pitch of the sleeve, the shape of the body — all of them build on many decades of knowledge, respecting them, treating them all with a light touch.