The Collaborative International Dictionary of English v.0.48:

Preventer \Pre*vent"er\, n.
   1. One who goes before; one who forestalls or anticipates
      another. [Obs.] --Bacon.
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   2. One who prevents or obstructs; a hinderer; that which
      hinders; as, a preventer of evils or of disease.
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   3. (Naut.) An auxiliary rope to strengthen a mast.
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   Preventer bolts, or Preventer plates (Naut.), fixtures
      connected with preventers to re["e]nforce other rigging.


   Preventer stay. (Naut.) Same as Preventer, 3.
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