And then there is what Texas Governor Ann Richards said about George Herbert Walker Bush: “Poor George. He was born with a silver foot in his mouth.”
“Thick as a whale sandwich.” – Blackadder
“Strong like ox, smart like tractor” has always been one of my faves.
I’ve always liked saying stuff like ‘not the brightest knife in the drawer.’ Just makes me laugh.
The butter has slipped off his pancakes.
Nothing is blowing thru the windmills of his mind.
Dumb as a bag full of turnips.
The water in his toilet is swirling the wrong way.
Sharp as a mashed potato sandwich
Not the fastest elk in the herd.
And when I was teaching, a colleague (who used that expression a lot) had a student write that someone was “Not the fastest alp in the herd.” Holy reflexive statements, Batman!
I’m also partial to “A few tacos short of a combo platter,” “Dumb as a bag of hammers” (mentioned elsewhere), and “Got a mind like a steel wool.”
Not the fastest elk in the herd.
And when I was teaching, a colleague (who used that expression a lot) had a student write that someone was “Not the fastest alp in the herd.” Holy reflexive statements, Batman!
I’m also partial to “A few tacos short of a combo platter,” “Dumb as a bag of hammers” (mentioned elsewhere), and “Got a mind like a steel wool.”
Not the fastest elk in the herd.
And when I was teaching, a colleague (who used that expression a lot) had a student write that someone was “Not the fastest alp in the herd.” Holy reflexive statements, Batman!
I’m also partial to “A few tacos short of a combo platter,” “Dumb as a bag of hammers” (mentioned elsewhere), and “Got a mind like a steel wool.”
How has noone mentioned “as thick as two short planks”??
I also like “a couple of roos loose in the top paddock” (already mentioned by phraser), and “a couple of TimTams short of a packet”.
But I think my favourite is “hasn’t got two brain cells to rub together” 
OH, I like that one, tritone!
I like this place.
www.herbison.com/canon/fulldeck.html
Couldn’t find his ass with two hands and a flashlight.
Dumb as a sack of hair.
Couldn’t find his ass with ass-finding radar.
-mdf
Mother liked to describe a collegue of hers thus:
“He couldn’t find his ass with both hands and a road map.”
A few sandwiches short of a picnic.
2 eggs short of a dozen
a couple cookies shy of a full pack
a few cards short of the deck…and he’s left the jokers in
a few swallows short of a summer
a few presents short of xmas