'Hard' in the Bible

I am sending you to them who are stubborn and obstinate children, and you shall say to them, ‘Thus says the Lord God.’

not to many peoples of unintelligible speech or difficult language, whose words you cannot understand. But I have sent you to them who should listen to you and pay attention to My message;

Behold, I have made your face as hard as their faces and your forehead as hard as their foreheads.