The word 胃口 is formed by combining the characters for "stomach" (胃) and "mouth" (口) to create a single concept that transcends the individual parts. It visually and conceptually connects the physical organs of ingestion, the mouth as the entry point and the stomach as the container, to metaphorically represent the overall desire and capacity for eating. This fusion effectively captures the holistic sense of one's "appetite", extending naturally to describe enthusiasm or interest for non-food things, as both the desire (mouth's taste) and the capacity to enjoy (stomach's holding) are inherent in the term.