"lema'anchem": Because of you this was done to me. "and He would not hearken to me": He did not accept my prayer. "And the L-rd said to me: 'rav lach. Presume to speak to Me no more about this thing.'" An analogy: A man makes a vow. Does he not go to his master for absolution of his vow? If the master makes a vow (affecting his disciple), it is the disciple's part to heed the words of his master. You, too, heed the words of your master. (Thus, "rav lach" ["You have a Master!"])
Variantly: "And the L-rd said to me: 'rav lach'" (i.e., you are a "teacher" in this thing) — You are an example for judges to say: If the great sage, Moses, was not forgiven for saying (Bamidbar 20:10) "Hear, now, you rebels," but it was decreed that he not enter Eretz Yisrael, then those who delay judgment and those who pervert justice, how much more so! And if Moses who was told (Ibid. 12) "You shall not bring this congregation into the land," did not refrain from imploring the L-rd for mercy — how much more so (should this be the pattern for) other men, (who were not told such a thing!) And if Chezkiah, who was told (II Kings 20:1) "Set your house in order, for you shall die and not live" did not refrain from imploring the L-rd for mercy, acting on the principle of "Even if a sharp sword is poised over a man's neck he should not refrain from imploring mercy," as it is written (Isaiah 38:2) "then Chizkiyahu turned his face to the wall and prayed to the L-rd" — how much more so other men!