thick as mince
(as) thick as mince
slang Remarkably stupid, dimwitted, or obtuse. Primarily heard in UK. Jen's new girlfriend is very nice, but she's thick as mince. The MP was caught on a hot mic calling his colleague "as thick as mince."
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
thick as mincemainly SCOTTISH
If someone is as thick as mince, they are very stupid. No point in expecting any real help from her department — most of them are as thick as mince. Well, what do you expect? She's as thick as mince.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012