San Francisco 49ers receiver Jauan Jennings was on the receiving end of a punch to the junk during "Monday Night Football" ... and he was so pissed about the move, he ended up fighting back after the game.
The attack below the belt went down late in the Niners' 20-9 victory over the Carolina Panthers on primetime ... when Jennings and defensive back Tre'von Moehrig were in each other's faces amid the action.
Jennings was visibly frustrated and in pain over the act ... and after the win was secured, he retaliated by swinging on Moehrig.