Idiom: As thick as two short planks

[Haihao]

English Idioms and Expressions, Advanced Level

ESL/EFL Test #15 "Teaching your Family to Drive", question 7

You'll be lucky if you get more than two words out of him because I'm afraid he's as thick as two short planks.

(a) very quiet
(b) very slow
(c) very wooden
(d) very stupid

English Idioms and Expressions, Advanced Level

ESL/EFL Test #15 "Teaching your Family to Drive", answer 7

You'll be lucky if you get more than two words out of him because I'm afraid he's very stupid.

Correct answer: (d) very stupid
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I really couldn't figure out what the idiom means. Could you give me the picture of it?

Thank you.
haihao

[Mister Micawber]

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Thick (casual) = lacking mental agility; stupid. 'Two short planks' (two wooden boards of considerable physical thickness) is merely a jocular addition to the basic meaning.
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Native English teacher at Mister Micawber's