self-contradiction - 自相矛盾

the idiom self-contradiction came from this old Chinese story :

A weapon seller in the market place boasted that his spears could pierce through anything in the world, and his shields can withstand any weapons.  A spectator asked him what happened if he used his spears to pierce through his shields.   Discovered his own contradiction,  he could not answer the question! 

But the word formed by putting the 2 Chinese characters (spear and shield) together has meant “contradiction” ever since, while all 4 characters mean “self-contradiction” 

自 =  self,       相 =  appear,  
矛 =  spear,     盾 =  shield     =>     矛盾 =  contradiction