Wiktionary
melon head
n. 1 (context idiomatic English) A dimwit, a fool 2 A melon-headed whale
Usage examples of "melon head".
One had a copper cauldron strapped upside down on his melon head in lieu of a helmet.
Oh, you wonderful windbag, you priceless melon head, you haven't the slightest idea what you're mixed up with!
After a little while Frank came back with a large, bald man in his fifties with pop eyes and a melon head and a voice so soft that it might have come from a sick child.
The first thing I thought about was Randy Briggs and how it would feel good to drag him by his little feet down the stairs of his apartment building, with his stupid melon head going bump, bump, bump on the steps.