SAP

to enervate or weaken the vitality of

deplete
SYN - exhaust, playout, run down, tire

excavate the earth beneath

a fool or nitwit

a person who lacks good judgment
SYN - fool, muggins, saphead, tomfool

"Even the thought of their relationship would sap his mood so he continued to stay busy and keep the thoughts at bay."

"Whenever he started playing video games it would sap his desire for learning and self-improvement."

"She didn't want to risk the sapping effects a look at the truth would produce."

"What was he doing in this class he was nothing but a sap."

"He couldn't believe that the office of the presidency was now being occupied by this sap."

"She knew that she had to do it herself since she didn't and shouldn't trust the judgment of the sap that was in charge."