justification or
reason that permits a statement or request to be legitimately made. Does not imply rightness, just basic acceptability.
I don't know whether she'll let me borrow her car after last time, but if I pay to have the blood removed from the upholstery at least I'll have a leg to stand on when I ask.
Most commonly used in the expression "[person/group/idea] doesn't have a leg to stand on"