Well then, my dear friend Euthyphro, do tell me, for my better instruction and information, what proof have you that in the opinion of all the gods a servant who is guilty of murder, and is put in chains by the master of the dead man, and dies because he is put in chains before he who bound him can learn from the interpreters of the gods what he ought to do with him, dies unjustly; and that on behalf of such an one a son ought to proceed against his father and accuse him of murder. How would you show that all the gods absolutely agree in approving of his act? Prove to me that they do, and I will applaud your wisdom as long as I live.
Under normal circumstances, a question like this would be taken as an insult. It is a simple question that disassembles Euthyphro's entire argument because it requires that he provide the impossible or he loses all credibility to his case. Socrates must know this because he taunts his respect as a reward if the impossible is achieved, knowing that it cannot be done and he will simply never have to hold this agreement. At least this is how I interpret it.