Dumber than Miss South Carolina 

When someone gives the answer to a different question because they are too dumb to understand the question they were asked.
Derived from the recent Miss America pageant when Miss South Carolina had the ultimate blonde moment.

Q. What is your opinion on the global warming issue?
A. I'm very happy the globe is warming up because then people won't have to use tanning beds as much so there won't be as much skin cancer.

That girl is dumber than Miss South Carolina

Dumber than Miss South Carolina by Brunette from the North August 10, 2009

Dumber Than Dirt 

A variation of "dumb as dirt" used in the graphic adventure video game Full Throttle developed by LucasArts.

Extremely stupid.

Likely conceived for its alliteration.

Ripburger, you're dumber than dirt!

Dumber Than Dirt by Carl Sackrider February 23, 2019

Dumber Than Snape 

One who is the dumbest of the dumb, dumber than anyone else by comparison.

Becky thinks that Boston is a state! Man, she's dumber than Snape.

Dumber Than Snape by The Goddamn Batcolin January 4, 2012

dumber than a box of brick shit

completly useless , waste of skin on a voyage to dipshit land.

That cop was dumber than a box of brick shit.
That guy really thought his wife got lost in oklahoma for six hours. He must be dumber than a box of brick shit.

dumber than a box of brick shit by pinhead77 December 15, 2008

Dumber Than a Bag of Hammers 

Speachless.

As unvocal as a stationary sack of primitive kinetic fastening implements

"My neighbor's chihuahua bothered me, so when they were on vacation I absconded with it, anesthesized it, removed the offending throat organ, and bribed the housesitter to keep her mouth shut. Now it is dumber than a bag of hammers and my neighbors are slightly puzzled."

Dumber Than a Bag of Hammers by UDPseudonym001 May 3, 2020

dumber than a sack of dirty underwear 

Can be said about someone seriously lacking brain activity.

Me- You'll never believe what Lil Kurt did today!
Evil Tim- What did he do? Did he try to marry a donkey again?
Me- No, it's even more idiotic than that... the boy tried to launch himself into orbit by supergluing some Roman Candles to his butt! He's in the hospital now for severe rectal burns.
Evil Tim- Man, he's dumber than a sack of dirty underwear...

dumber than a sack of dirty underwear by Shawn B. January 15, 2007

Dumber than a bag of dicks 

Stupid

She was "dumber than a bag of dicks" when she drove around drunk because that is stupid.

Dumber than a bag of dicks by Les mcclary February 24, 2019