The antonyms for the word "hack up" include repair, fix, mend, heal, and restore. These words signify the act of putting things back together in a constructive manner. When we hack up something, we are often destroying it without any intent to rebuild. However, with the antonyms, the focus is on repairing the damage, rather than causing more harm. For instance, if something is damaged or broken, we can fix it by mending it. Similarly, when our body is injured or wounded, we heal ourselves by taking care of the injury with the intention of restoring the original state. Hence, the antonyms for hack up are quite useful in situations where a constructive approach is required.