Meaning of (AS) THICK AS TWO (SHORT) PLANKS (OR AS A PLANK) BRIT. in English

(AS) THICK AS TWO (SHORT) PLANKS (OR AS A PLANK) BRIT.

(as) thick as two (short) planks (or as a plank) Brit.

informal

very stupid.

→ thick

Concise Oxford English vocab.      Сжатый оксфордский словарь английского языка.