Plumhead noun say plum+head.The adjective is plumheaded.
Could be anything you want it to be. I tend to use it to refer to a total wanker/tosser or twat or whichever. It is designed to be a humourous euphemism to create global piece and prevent people from taking life too seriously and doing seriously destructive things i.e. killing everyone.
It came about as a spontaneous thought by me Jim the inventor of the word.
George Bush is a Plumhead.