The word **忙于** is a compound formed by combining the adjective **忙** (máng, meaning "busy") with the prepositional particle **于** (yú, which here indicates "in regard to" or "in the sphere of"). Grammatically, **于** attaches to the adjective, transforming it into a preposition-like phrase that means "to be busy with/doing (something)." It follows a common Chinese structural pattern where **于** links a state or action to its object, creating a fixed, inseparable unit that functions as a verb phrase requiring a following object (e.g., 忙于工作, "busy with work").