The word "干活" is formed by directly combining two conceptual ideas: the character "干", which conveys the core action of "doing" or "working upon", with the character "活", which represents the object or substance of that action, namely "work" or "a task" itself. It is a straightforward, almost visual pairing where the first element applies its force to the second, creating the unified concept of "to work" or "to do labor", mirroring the intuitive process of an action being directed at its tangible target.