The Collaborative International Dictionary of English v.0.48:

Horner \Horn"er\, n.
   1. One who works or deal in horn or horns. [R.] --Grew.
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   2. One who winds or blows the horn. [Obs.] --Sherwood.
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   3. One who horns or cuckolds. [Obs.] --Massinger.
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   4. (Zool.) The British sand lance or sand eel (Ammodytes
      lanceolatus).
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