‘clotpole’, also spelled ‘clatpole’, is defined as an idiot, a foolish or very stupid person, someone who is oblivious to what is going on. It is pretty much exclusive to Merlin and Arthur. It’s composed of "clot" meaning fool or oaf and "pole" referring to the male genitalia. In Elizabethan slang, it means 'wooden head' or 'block head'.

clotpole is a very old insult, the early form of clodpate. Both can be found in the OED, and essentially mean "dirt head." Clod and clot both mean dirt or earth, pole and pate both refer to someone's noggin (pate more obviously than pole).