Lion Heart

2 Samuel 17:10
Then even the bravest soldier, whose heart is like the heart of a lion, will melt with fear, for all Israel knows that your father is a fighter and that those with him are brave.

Ahithophel advised Absalom to make a quick and ruthless attack on his father before David got too far. The plan might well have worked. So Hushai, who had been planted to help David, suggested an alternate plan to buy time for the king.
First he played on David's reputation as a warrior, portraying the results if the hasty attack went badly. Here heart is courage, spirit, energy, and the image is similar to the melting hearts of other verses. However, it is the soldier himself who will melt before David's counterattack, though he have the courage of a lion.
Hushai is dealing skillfully in word pictures here. Melting soldiers, lion hearts...
Next he plays to Absalom's ego, picturing the world's largest army, led by Absalom, utterly destroying David and everyone with him. How much more appealing than a quick and dirty assassination mission!
Absalom went for it.