to jump the gun


From The Collaborative International Dictionary of English v.0.48:

Jump \Jump\, v. t.
   1. To pass over by means of a spring or leap; to overleap;
      as, to jump a stream.
      [1913 Webster]

   2. To cause to jump; as, he jumped his horse across the
      ditch.
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   3. To expose to danger; to risk; to hazard. [Obs.]
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            To jump a body with a dangerous physic. --Shak.
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   4. (Smithwork)
      (a) To join by a butt weld.
      (b) To thicken or enlarge by endwise blows; to upset.
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   5. (Quarrying) To bore with a jumper.
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   To jump a claim, to enter upon and take possession of land
      to which another has acquired a claim by prior entry and
      occupation. [Western U. S. & Australia] See Claim, n.,
      3.

   To jump one's bail, to abscond while at liberty under bail
      bonds. [Slang, U. S.]

   To jump the gun, to begin to run (in a footrace) before the
      starting gun has fired; hence, (fig.) to begin any
      activity before the designated starting time.
      [1913 Webster +PJC]
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