Was Thick as Two Short Planks?

The British-English phrase (as) thick as two short planks means very stupid. The humorous arbitrary comparison with two short planks gives emphasis to the adjective thick, meaning stupid.

What does as thick as two short planks mean?

UK slang. very stupid: He's a very skilled football player but he's as thick as two short planks.

Where does as thick as two short planks?

Two short planks are even thicker. The expression 'as thick as two short planks', which is sometimes given in the variant form 'as thick as two planks', is of UK origin and dates from the 1970s. The earliest example of it that I know of in print is in Gordon Honeycombe's novel Adam's Tale, 1974.