a leg to stand on
Proof, defense, or support for one's argument or position. Often used in the negative to mean the opposite. The senator's company avoided paying millions in taxes over the years, so she doesn't have a leg to stand on when she criticizes tax dodgers. The truth is that the prosecution hasn't got a leg to stand on, now that their key witness was discredited in court.
a leg to stand on
Slang A justifiable or logical basis for defense; support: He doesn't have a leg to stand on in this debate.