The Comprehensive Aramaic Lexicon

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