The word 后背 is formed by combining two meaningful characters that each describe an aspect of the concept. The first character, 后 (hòu), carries the core idea of "rear" or "behind". The second character, 背 (bèi), fundamentally means "the back" of something, often the human torso. By placing them together, the concept is visually and logically built: 后 specifies the spatial orientation, and 背 specifies the body part, resulting in the combined meaning of "the rear side of the back", essentially, the back (of the body).