updated list

"one brick shy of a load."
"The lights are on, but nobody's home."
"elevator does not go to the top"
"a few crackers short of a box"
"a few pickles short of a barrel"
"a sandwich short of a picnic"
"A couple french fries short of a Happy Meal"
"Not playing with a full deck"
"All his/her oars are not in the water."
"Kangaroos in the top paddock"
"If he/she had half a brain their head would rattle"
"He/she has two brain cells, one's out looking for the other"
"in a battle of wits, he/she is unarmed"
"His/her brain is like a camera, one bright flash and it's over."
"Bats in the belfry"
"lost their marbles"
"Half a bubble off plumb"
"A few beans short of a burrito"
"Not the brightest crayon in the box, are we?"
"not the sharpest knife in the drawer"
"a demitasse cup would fit his head like a sombrero"
"he would be out of his depth in a parking lot puddle"
"if you give him a penny for his thoughts ,you get change back"
"forgot to pay his brain bill"
"an early example of Peter's Principal"
"Diagonally parked in a parallel universe."
"Doesn't just know nothing; doesn't even suspect much."
"Goalie for the dart team."
"Immune to caffeine and all other stimulants."
"when God was passing out brains, he/she thought he said 'rain' and hid
under a toadstool"
"He/She's off the beam"
"He/She's a quart low"
"He/She's a loaded bucket with a hole in the bottom"
"He/She's two sheets to the wind in a calm"
"Somwhere out there is a villiage being deprived of an idiot"
"That boy/gal is from third eyeball country, ya think?"
"That boy/gal's been out in the sun too long, brain's fried."
"That boy/gal's cerebrum is the circumfrence (sp?) of the anus of an
arachnid."
"Couldn't count his (insert body part) and get the same count twice."
"Doesn't have all his/her dogs barking"
"Couldn't organise a piss-up in a brewery"