The Collaborative International Dictionary of English v.0.48:

Jetty \Jet"ty\, n.; pl. Jetties. [F. jet['e]e a pier, a jetty,
   a causeway. See Jet a shooting forth, and cf. Jutty.]
   [1913 Webster]
   1. (Arch.) A part of a building that jets or projects beyond
      the rest, and overhangs the wall below.
      [1913 Webster]

   2. A wharf or pier extending from the shore.
      [1913 Webster]

   3. (Hydraul. Engin.) A structure of wood or stone extended
      into the sea to influence the current or tide, or to
      protect a harbor; a mole; as, the Eads system of jetties
      at the mouth of the Mississippi River.
      [1913 Webster]

   Jetty head (Naut.), a projecting part at the end of a
      wharf; the front of a wharf whose side forms one of the
      cheeks of a dock.
      [1913 Webster]