In his portrait, Percy is wearing the appropriate service headgear, a khaki cap, with a reinforced peak and the rim of its crown stiffened with wire. Just below the insignia you can see a narrow leather band, held onto the cap by two metal fasteners, and capable of being lowered to serve as a chin strap.
It is said that in the field, soldiers sometimes removed the stiffening wire to make the cap soft and foldable, and to give the impression of having served for some time. To do so was against regulations. According to some sources, the resulting crushed cap was called a gor-blimey, supposedly because when it slipped forward over the eyes, it evoked that exclamation of surprise from its British wearers. (1) Other sources tell us that a gor-blimey was in fact a British cap with ear flaps, which could be tied up if not needed into an “odd, baggy pile on the head” and therefore “looked a little goofy.”(2)
(2) Tim Cook. At the Sharp End.Canadians Fighting the Great War, 1914-1916. Vol 1. 2007.79