The idiom 不胫而走 is formed by directly combining the characters 不 (not), 胫 (shin or leg), 而 (a classical conjunctive particle meaning "yet" or "but"), and 走 (to run). The literal sequence "without legs yet runs" creates a metaphorical expression where the absence of a physical means of movement contrasts with the action of running, thus conveying the idea that information or news spreads rapidly on its own.