(AS) THICK AS TWO (SHORT) PLANKS (OR AS A PLANK) BRIT.
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.
informal
very stupid.
→ thick
Concise Oxford English vocab.
Сжатый оксфордский словарь английского языка.
2004