The word "不经意" is a fixed compound formed through a logical syntactic combination: the negative adverb "不" (bù, meaning "not") modifies the verb-object phrase "经意" (jīngyì, meaning "to pass through one's mind" or "to pay attention to"). Literally, it means "not passing through the mind," which evolved into its modern figurative meaning of "unintentional," "unconscious," or "without noticing." It functions as an adverb or adjective describing an action done casually, accidentally, or without deliberate thought.