The Chengyu 自相矛盾 (zì xiāng máo dùn) literal means to be both the spear (矛, máo) and shield (盾, dùn). The meaning is to contradict oneself or means inconsistent.
There once was a weapons dealer in ancient China who had goods to sell. The weapons dealer approached a quiet town and started promoting his wares, hollering as he rang his gong: “Gather round!”
“I possess a fine spear - a spear so strong, it can penetrate the strongest armor. A must for fighting off the Huns!” The townsfolk stood quietly, not quite believing the salesman.
Sensing hesitation, the disheartened merchant continued with a new spiel: “Now look here instead: a strong, sturdy shield that protects against all incursions. You want it away? This shield will keep it at bay!”
At this point, one of the villagers asked: “What happens if I take your spear and ram it into your shield? Would the spear snap? Or would the shield shatter?”
The villagers realized how silly the merchant was. And thus, amidst laughter and ridicule, the merchant shamefully fled the town.