Simply, the expression "You've got no balls" can stand for:
You have no self-respect, you are a coward, you can't face your problems like a man, etc.
After the expression is told to someone, he has to prove otherwise- do what he was told to do, or simply say:
"I do have balls. You wanna see them?"
-No, it will get me into trouble.
-You've got no balls! Go and face him like a man!