Six Favorite Could Nots
Couldn't hit sand if he fell off a camel.
Couldn't hit the broad side of a barn if he were standing inside.
Couldn't pour water out of a boot with instructions on the heel.
Couldn't think/pee his way out of a paper bag.
Couldn't write dialog for a porno flick.
CPU is always in powersave mode.
CPU not connected to the bus.
Cranial cavity filled with neutronic matter. (Really dense.)
Cunning as a dodo bird.
Cursor's flashing but there's no response.
Dealing with him is less fun than going to the dentist.
Dealing with him is one angst worse than a blind date.
Deep as her dimples / reflection in a mirror.
...any more to add?