If you are in a situation where you 'don't have a leg to stand on' you are in a precarious situation where you don't have support, evidence or proof to support your position.

1. There's no way the defendant isn't going to jail at the end of this trial. He doesn't have a leg to stand on!
2. He claims that she hit him but without a shred of evidence and no witnesses and with his track record,he doesn't have a leg to stand on.

1. The company don't have a leg to stand on when it comes to criticizing the business ethics of their competitors as they have been engaging in shady practices for years!
2. I thought about suing my former employer for unfair dismissal, but without clear proof of a breach of contract I wouldn't have a leg to stand on.