Not have a leg to stand on
Definition 1 of 1
Pro Tip 1/3Metaphorical Usage
The phrase is used metaphorically to express a lack of evidence or support in an argument. 
Without witnesses, your claims about seeing a UFO won't have a leg to stand on.
Pro Tip 2/3Legal Contexts
Commonly used in legal discussions where evidence is crucial for a valid argument. 
In court, without concrete evidence, the defense doesn't have a leg to stand on.
Pro Tip 3/3Colloquial Expression
It's a colloquial saying, so it's best used in casual or less formal conversations. 
He tried to get a refund without a receipt, but he didn't have a leg to stand on.