a lie has no legs

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. 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." I'm not surprised their key witness was discredited in court, what with that clearly fictitious story she told. A lie has no legs, after all. Mary Anne has always been super sweet to me, so I don't believe the bad things they're saying about her. A lie has no legs, as far as I'm concerned.