In the context of contract law, debt collection and civil litigation, the term judgment proof is commonly used to refer to defendants or potential defendants who are financially insolvent, or whose income and assets cannot be obtained in satisfaction of a judgment.
Being "judgment proof" is not a defense to a lawsuit. If sued, the defendant cannot claim being "judgment proof" as an affirmative defense. The term "judgment proof" instead refers to the inability of the judgment holder to obtain satisfaction of the judgment.
If a plaintiff were to secure a legal judgment against an insolvent defendant, the defendant's lack of funds would make the satisfaction of that judgment difficult, if not impossible, to secure.