>>We had a "not all of his sheep on count", which initially meant a sad or just downbeat person, i.e. as happy as a shepherd who lost a sheep. That somehow got mixed up with "missing a plank" (which later expanded into "missing a whole door", "missing a lumberyard") i.e. there's a hole and there's a draft through his head... so in the end you get "not all of his on count" (sheep or planks got omitted) to simply mean the guy is understaffed in the brains department.
>
>A few americanisms I have heard for this meaning:
>
>Lights are on, but no one's home.
>Two beers short of a six-pack.
>Not the sharpest tack in the box.
>Not the brightest bulb on the tree. (refering to a Christmas Tree)
>Bats in the bellfry. (sp)

And yet more....

Three bricks shy of a load
A few fries shy of a Happy Meal
Not the sharpest knife in the drawer
Elevator doesn't go all the way to the top floor