thick as shit
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(as) thick as a shit
vulgar slang Remarkably stupid, dimwitted, or obtuse. Primarily heard in UK. I smoked a lot of marijuana when I was in high school, so I turned out thick as a shit by the time I finally graduated. Jen's new girlfriend is very nice, but she's as thick as a shit.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
thick as shitBRITISH, INFORMAL, VERY RUDE
If someone is as thick as shit, they are very stupid. We can only conclude that these people are either thick as shit, blinded by racism, or both.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012