Useless as a bent nail.

Totally lacking in the ability to get the job done. Derived from carpenter slang to refer to the metalurgical properties of a nail deformed and bent by extreme resistence in the material being nailed, or due to the inexperience or lack of skill by the person striking the nail. Despite streightening, once the nail is used again, it will tend to deform and bend, rendering it useless.

I sent Bob out to clinch the deal, and instead the client called to cancel the order! I swear, Bob is "Useless as a bent nail."

Useless as a bent nail. by Jakatola August 2, 2009