thick as two planks
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thick as two planksor
thick as two short planksBRITISH, INFORMAL
If someone is as thick as two planks or as thick as two short planks, they are very stupid. His people regarded him as a great and wise king. In fact he was as thick as two planks.
thick as two (short) planksvery stupid. informal
Variants of this expression include thick as a plank and thick as a brick . There is a play on thick in its basic sense ‘of relatively great depth from side to side’ and its colloquial sense ‘stupid’.