The word 副作用 is formed by combining three meaningful characters into a single conceptual unit: 副 (fù), meaning "secondary" or "deputy"; 作 (zuò), meaning "to act" or "action"; and 用 (yòng), meaning "use" or "effect". It is constructed conceptually by sequentially layering these ideas, first establishing a primary, intended action or use (implied), then specifying this as the secondary (副) action/effect (作用) that arises alongside it. Thus, the compound directly and intuitively encapsulates the notion of a "by-effect" or an ancillary consequence occurring beside the main intended purpose.