What does "hornbeam" mean?

Hornbeam: A tree of the genus Carpinus (C. Americana), having a smooth gray bark and a ridged trunk, the wood being white and very hard. It is common along the banks of streams in the United States, and is also called ironwood. The English hornbeam is C. Betulus. The American is called also blue beech and water beech. Hop hornbeam. (Bot.) See under Hop.