Most casehardened refers to something that is extremely hardened or toughened. The antonyms of this phrase would be softening, weakening, and tenderizing. A substance that is most casehardened is difficult to break or bend, making it durable and strong. Therefore, the opposite of this would be fragile, brittle, and delicate. In terms of behavior, someone who is most casehardened may be seen as unyielding or stern. The antonyms of this would be lenient, flexible, and accommodating. Most casehardened can also refer to a person's attitude or feelings, so antonyms could include sensitive, compassionate, and understanding.