The word 镇静剂 is formed by logically combining three meaningful characters: 镇 (to suppress/quell), 静 (calm/still), and 剂 (agent/substance). The first two characters unite to create the core concept of "calming" or "sedation" (镇静), which then modifies the third character, 剂, which specifies the category. Thus, the compound directly and descriptively denotes a "calming agent" or a substance that induces sedation, building its meaning from the sequential contribution of each component's inherent sense.