TgJ Jer2:30 :לְמַגָן אַלקֵיתִי יָת בְנֵיכוֹן אֻלפָן לָא קַבִילֻו‏ : you have whipped your children in vain: they have not accepted instruction
See the entire entry for lmaggān,lmaggānā adv. in vain