To sap something is to drain or deplete something over time. If you sap a maple tree, you drain the liquid inside it to make maple syrup. But if you sap a person of strength, you've rendered him defenseless.
Whether used as a noun or verb, sap is rarely a good thing. If your energy or will is sapped, it’s not meant lightly; it means you have been exhausted of all your reserve energy, you’re reduced to a shell. If someone calls you "a sap," it suggests you lack strength and character. And if you get sap — the sticky liquid inside a tree — on your hands, good luck getting it off in the middle of a forest without a bar of soap and running water. Yuck.