A post in this thread reminded me of a thread I was wnting to start. I need some new “dumb as a” expressions. The classics are of course “dumb as a bag of hammers” and “dumb as a box of rocks” (or “box of hair” as an alternate). But I need some new ones.
So far the best among several lame attempts is “dumb as a suitcase full of hats.” I know y’all can do better than that. Little help?
Trying to avoid the “s dumb as a (container) of (objects)” the only thing I can come up with is “As dumb as a walrus BASE jumping without a parachute.”
How about “As dumb as a star of popular TV show leaving it to pursue a career making intermittent and uniformly terrible films.” Too unwieldy?
not quite in the same form:
“not the sharpest tool in the box”
or
“not the brightest bulb in the hallway”
As dumb as a dumb person doing something really…dumb.
As dumb as a quilted bikini.
As dumb as a handle on a bowling ball.
As dumb as a leather parachute.
As dumb as a tap dancer in a minefield.
Dumb as a stump.
That boy’s about as sharp as a sack full of wet mice.
dumb as a teenager in a whorehouse
sharp as a ball peen hammer
the shortest crayon in the box
“too dumb to pour piss out of a boot with the instructions printed on the heel” Lyndon Johnson
Dumb as a fence post.
Not really a dumb as but I like as useless as tits on a boar hog too.
**Dumb as a box of mimes. **(Playing on “dumb” also meaning mute)(And the fact that mimes love that dumb air-box)
Dumb as a screen door on a submarine (It alliterates better as “stupid as a screen door. . .”)
Dumb as a brick wall and twice as funny
Dumb as a Ken doll in a porn film
Not a ‘dumb as’ but feel free to use this as necessary…
“tighter than a preacher’s cock in a heffers ass”.
Works well when removing nails or unscrewing the lid on the pickle jar. 
Dave
Then of course you have the “(blank) short of a (blank)” series:
2 tacos short of a combination plate
2 beers short of a six pack
a sandwich short of a picnic
and so on…
[Beavis voice]
Heh heh,
you said box of hair.
Heh heh.
[/Beavis voice]
Dumb as a push-up pop.
If dumb was dirt, he’d cover about an acre.
If brains were leather, he couldn’t saddle a flea.
He’s 'bout as sharp as a mashed potato.
He’s got an IQ of 2 and it takes 3 to grunt.
He’s a french fry short of a Happy Meal.
If brains were gas,he couldn’t drive a go-cart around a cheerio.
Dad always told me I was “dumber than a coalbucket”. And then he’d tell me I “couldn’t find my butt with both hands and a map”. I love my Dad! (no, really!)
I personally like “daft as a brush”.
Sharp as a marble
My dad always used to say that some people “couldn’t find their ass with both hands and a flashlight.” I still use that one to this day
Good Britishism: “thick as two short planks.”
I heard this used a few years ago while in Arkansas:
“That boy’s dumber than a turd in a bucket. If you shove his brain up a gnats ass, it will rattle like a BB in a box-car.”
Dumber than a house plant.
My favorite: Dumber than a ditch carp.