A fatally flawed argument is one from which nothing can be salvaged. Given that all arguments seek to establish something, an argument which is fatally flawed has such critical problems with its structure, premises, reasoning, or any combination of these that it is simply impossible to rework the argument to be effective. This can be contrasted with flawed arguments, which require structural reworking, and unsound arguments, which are based on incorrect premises.
A classic example of a fatally flawed argument is the ontological argument. Due to equivocation between an existent God and a conceptual God, the argument is flawed. However, due to its syllogistic and analytic, strong deductive nature, it is impossible to represent the anything recognisable as the ontological argument in a new form, and the flaw is fatal.