Because a lie has nothing to support it ("no legs" to stand on), it is unsustainable and can be easily recognized and dismissed as a lie.
因为一个谎言无关,支持它(“无腿”站立),它是不可持续的,能够容易地识别并斥为谎言。
A: "I'm really worried that this rumor about me is going to reach the whole school." B: "No way—a lie has no legs, so the truth will definitely prevail."