Is He doesn't have a leg to stand on a cliche?

Answer 1

Yes it is a cliche. It's a metaphor, of course. The image is of something without support, an unsupportable or shaky argument as of someone trying to stand on shaky legs. At one time, it was fresh and the imagery striking. Now, we hardly think about the imagery as the phrase itself is automatically read as meaning, 'unconvincing'. In general, cliches like this are best avoided as they tend to be untranslatable. In the original context of the phrase, the meaning would have been obvious. It is far better to think of a fresh alternative or failing that, use simple English such as, "His logic is flawed."

Answer 2

Yes, "He doesn't have a leg to stand on" is a cliche that means someone lacks a strong argument or evidence to support their position.