Numnuts or num-nuts is a slang term that is used to depict someone who is a constant source of trouble. Usually an individual who screws up, or constant makes mistakes. Someone who botches a job, event, or situation.
"Good job, numnuts!"
"You guys are a bunch of #%^&!$$ numnuts! Hit the ground and give me 50!" (a drill sergeant to his platoon of Army trainees.