pigheaded

Pigs are stubborn...even if you force them not to eat shit, they will eat it stubbornly.....

pigs are stupid and they dont do any work ;) - hence obstinate

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Pigs are big-headed and stubborn. When you clean them, they go right back and get all dirty, muddy and murky. Similar to a pigheaded person who is hard to change or correct and goes right back to status quo after correction.