The word "随手" is formed by directly combining the character "随", meaning to follow or along with, and "手", meaning hand, to create a literal composition meaning "following the hand" or "with the hand". This structural pairing yields an adverbial function that conveys performing an action casually, spontaneously, or as an incidental motion of the hand, as seen in common uses like "随手关门" (close the door without special effort).