One Short of A Six Pack
A term used to describe a person who is not all there. A person missing a marble or two - a few percentages off from being full blown retarded.
"I can't believe that guy just walked into that glass door - what is he...one short of a six pack?"
one beer short of a six pack
Someone who lacks common sense. Not quite mentally competent.
You're one beer short of a six pack. You have no idea what you're talking about.