Dirt Dumb 

Basically, the dumbest of the dumbest. Dumb as dirt.

Loyd is dirt dumb. He dropped his toilet paper roll in the toilet and put it in the microwave to dry it and burned the entire goddamn trailer park down. Dirt dumb.

Dirt Dumb by pdxjohnny99 March 27, 2016

dumb as dirt 

Being extremely stupid or gullible.

The Drill Instructor said Private Jones is as dumb as dirt.

dumb as dirt by DarkLurker May 1, 2005

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