also hammer-head, 1560s, "head of a hammer," from hammer (n.) + head (n.). From 1796 (American English) in reference to a kind of shark, so called for its broad, transverse head. The animal is referred to as hammer-headed shark from 1752 and hammer-fish from 1745. The older name for it was balance-fish; there was a full specimen and a head of another under that name in the Royal Society Museum by 1681:
He hath his Name not unaptly from the ſhape of his Head, very different from that of all other Fiſhes, being ſpread out horizontally, like the Beam of a Balance; his eyes ſtanding at the two extremes, as the iron Hooks do at the end of the Beam. He grows sometimes to the length of four or five yards: but this is a young one. [Nehemiah Grew, M.D., "Catalogue & Deſcription Of the Natural and Artificial Rarities Belonging to the Royal Society And preſerved at Greſham Colledge. Whereunto is Subjoyned the Comparative Anatomy of Stomachs and Guts. By the ſame author" London, 1681 ].