The word 善于 is formed through the conceptual fusion of its two component characters: 善, which carries the inherent meaning of being good, skilled, or proficient, and 于, a classical character that originally functioned to indicate a direction, target, or relationship. When combined, 于 acts not as an independent word but as a grammatical particle that attaches to and extends the core meaning of 善, transforming it from a static quality ("goodness" or "skill") into a directed capacity or relational state, specifically, the state of being skilled in relation to or in the realm of a following action or domain. Thus, the formation creates a single unit meaning "to be good at" by linking inherent aptitude (善) to a specific field of application (via 于).