(as) thick as mince British simile
Extremely stupid
After the test, I found myself as thick as mince to make such a silly mistake.
Though he knew she was a gold digger, he gave her all his money. He was as thick as mince.
She is thick as mince to love that bad man.
Very stupid, or silly
Stupid or crazy
To be extremely stupid; not to be alert
(n): a fool person,
(adj): stupid, foolish, silly
The origin of this slang is not clear.
Someone or something that tends to be unpredictable
Harry was always the joker in the pack at school.