dumb as a post

Translations
глупый как пробка
He is as dumb as a post when it comes to math.
Он глупый как пробка, когда дело касается математики.
Definitions
Extremely unintelligent or lacking in common sense; used to describe someone who is perceived as very slow-witted or incapable of understanding simple concepts.
Words
post

noun
почта
dumb

adjective
немой