solid, fixed

固 is composed of 囗 (encircle) and 古 (ancient). 囗 is a pictograph of an enclosure or fence, forming an ideogram for establishing a boundary. 古 is a pictograph of a shield, which originally carried the intrinsic sense of being hard, solid, and durable. As 古 later evolved to take on derivative meanings, 固 was formed by surrounding it with 囗 to preserve and reinforce its original sense—firm, solid, and secure.