(as) thick as two short planks
US /əz θɪk əz tuː ʃɔrt plæŋks/
UK /əz θɪk əz tuː ʃɔrt plæŋks/

1.
非常に頭が悪い, とんでもなく愚か
very stupid
:
•
He's (as) thick as two short planks, he never understands anything.
彼は非常に頭が悪い、何も理解できない。
•
You'd have to be as thick as two short planks to believe that story.
その話を信じるなんて、よっぽど頭が悪いに違いない。