Stubborn and unwilling to change your mind, even when you're wrong; very determined in a mulish way.
Compound from 'pig' and 'headed,' comparing human stubbornness to a pig's stubborn nature. The term dates back to at least the 1700s in English.
Ironically, pigs are actually quite intelligent and trainable animals—but this insult stuck because of pigs' reputation for charging blindly at obstacles rather than going around them!
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