The Collaborative International Dictionary of English v.0.48:

Jetty \Jet"ty\, a.
   Made of jet, or like jet in color.
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         The people . . . are of a jetty.         --Sir T.
                                                  Browne.
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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.]
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   1. (Arch.) A part of a building that jets or projects beyond
      the rest, and overhangs the wall below.
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   2. A wharf or pier extending from the shore.
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   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.
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   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.
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The Collaborative International Dictionary of English v.0.48:

Jetty \Jet"ty\, v. i.
   To jut out; to project. [Obs.] --Florio.
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WordNet (r) 3.0 (2006):

jetty
    n 1: a protective structure of stone or concrete; extends from
         shore into the water to prevent a beach from washing away
         [syn: breakwater, groin, groyne, mole, bulwark,
         seawall, jetty]