A stupid or foolish person; a blockhead or dull-witted individual.
From 'clog' (something heavy and unwieldy) plus 'head,' using a common English insult pattern (see blockhead, loggerhead, bonehead). The metaphor compares a thick skull to a heavy piece of wood.
English has an amazing collection of insults based on the 'head' pattern—blockhead, loggerhead, pothead, hotheaed—showing how creative people are about insulting each other's intelligence by comparing their brains to inanimate objects!
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