Hammerhead used to be a soldier for the mob... dumb muscle, a leg breaker. But then he survived a near-fatal head injury. He had most of his skull replaced with metal, and the rest of his body augmented with cybernetics. I've seen him run head-first into a brick wall and smash it to pieces. Wrap a machine gun into a pretzel. He's also hard-headed in the sense that he's stubborn... and that's how we'll get him.
Hammerhead is referenced in two of Detective J. DeWolff's audio journal entries (Jean DeWolff's Journal Part 02 & 03), in The Amazing Spider-Man 2.
Hammerhead was originally the surname of this mob boss and then taken on as his street name after suffering a head injury where he had to have an operation on his head operation where missing parts of his skull were replaced with metal giving his head a strange shape. He acts and dresses like a 1920's gangster.