The idiom 无影无踪 (wú yǐng wú zōng) is formed through a parallel and repetitive four-character structure common in classical Chinese, where 无 (wú) meaning "without" is repeated to negate two concrete nouns: 影 (yǐng, "shadow") and 踪 (zōng, "trace" or "footprint"). This symmetrical "A无B无C" pattern creates a strong rhetorical emphasis, combining two images of intangible evidence—a visual shadow and a physical track—to convey a comprehensive and vivid state of complete disappearance, leaving absolutely no sign or clue behind.