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Somebody is who is beyond stupid in general or at certain tasks. They are so dumb that they compare to a plank of wood in terms of brain usage.

In fact the plank of wood will usually solve the task before the person.
Jack: So did Alan manage to fix that rusty gate?

Bill: Nah man that lads as thick as a plank.
by MrWeldon February 06, 2018
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