Tony Thorne, The Dictionary of Contemporary Slang (1990) identifies plank in this sense as specific to British English: plank 1 British a dull-witted person, someone who is as 'thick as two short planks'.

What is the meaning of the word planks?

planked; planking; planks. Definition of plank (Entry 2 of 2) transitive verb. 1 : to cover, build, or floor with planks. 2 : to set down forcefully —usually used with down.

What does plank mean in the UK?

British English: plank NOUN /plæŋk/ A plank is a long, flat, rectangular piece of wood.