The word "嘿嘿" is formed through the process of reduplication, where the single syllabic character "嘿" (hei), which phonetically mimics a soft laugh or chuckle, is simply repeated. This doubling creates a harmonious, rhythmic sound that amplifies the original meaning, transforming a simple exhalation of sound into a more vivid, often playful or sly, representation of laughter or a vocal reaction. The formation is purely phonetic and aesthetic, using repetition to enhance expressiveness and convey a specific tone or attitude directly through its sound.