The word "龇牙咧嘴" is formed by combining two descriptive phrases: "龇牙" meaning to bare teeth, and "咧嘴" meaning to grimace with the mouth. The character 龇 incorporates the radical 齿 for tooth, specifying the action of exposing teeth; 牙 directly means teeth; 咧 includes the radical 口 for mouth, indicating grimacing; and 嘴 means mouth. Together, these characters sequentially depict the physical act of showing teeth and twisting the mouth to express a intense reaction.