helmet, basin

盔 is composed of 灰 (ashes) and 皿 (vessel). Thus, 盔 forms an ideogram representing a vessel used to hold ashes, with its original meaning being a basin or bowl. Because such a basin had to be sturdy to withstand the heat of fire, placing it upside down over the head served as effective protection; hence, 盔 extended to denote a helmet or protective headgear.