What or who is a "lunkhead"?

"Lunkhead" is someone who will fail to transmogrify you back into a boy from a tiger but instead transmogrified you into a vile-awful amphibian using a transmogrifier machine such as a cardboard box stated in no unclear terms on the outside that it is a "transmogrifier".

At times the following colorful words, omitting the more potent one, can take its place:

dunce, dunderhead, numskull, blockhead, bonehead, hammerhead, knucklehead, loggerhead, muttonhead.