The word "发脾气" is formed through a vivid conceptual metaphor rooted in traditional Chinese medicine. It literally breaks down to "issue" (发) "spleen" (脾) "qi/energy" (气), visualizing anger as the physical release of a harmful, pent-up energy originating from the spleen, an organ historically linked to emotions. Thus, the term paints a picture of an emotional outburst as a kind of internal, physiological eruption that is forcibly expelled from the body.