Definition of "hammerhead"
(slang) A stupid person, a dunce.
Quotations
[The butler] joined us with a telegram for Bobbie on a salver. From her mother, I presumed, calling me some name which she had forgotten to insert in previous communications. Or, of course, possibly expressing once more her conviction that I was a guffin, which, I thought, having had time to ponder over it, would be something in the nature of a bohunkus or a hammerhead.
1960, P. G. Wodehouse, chapter V, in Jeeves in the Offing