The Collaborative International Dictionary of English v.0.48:

backfire \backfire\, back fire \back fire\
   1. A fire started ahead of a forest or prairie fire to burn
      only against the wind, so that when the two fires meet
      both must go out for lack of fuel.
      [Webster 1913 Suppl.]

   2.
      (a) A premature explosion in the cylinder of a gas or oil
          engine during the exhaust or the compression stroke,
          tending to drive the piston in a direction reverse to
          that in which it should travel; also called a knock
          or ping.
      (b) an explosion in the exhaust passages of an internal
          combustion engine.
          [Webster 1913 Suppl. +PJC] Backfire

The Collaborative International Dictionary of English v.0.48:

Backfire \Back"fire`\ Back-fire \Back"-fire`\, v. i.
   1. (Engin.) To have or experience a back fire or back fires;
      -- said of an internal-combustion engine.
      [Webster 1913 Suppl.]

   2. Of a Bunsen or similar air-fed burner, to light so that
      the flame proceeds from the internal gas jet instead of
      from the external jet of mixed gas and air. --
      Back"-fir`ing, n.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

backfire
    n 1: the backward escape of gases and unburned gunpowder after a
         gun is fired [syn: blowback, backfire]
    2: a loud noise made by the explosion of fuel in the manifold or
       exhaust of an internal combustion engine
    3: a fire that is set intentionally in order to slow an
       approaching forest fire or grassfire by clearing a burned
       area in its path
    4: a miscalculation that recoils on its maker [syn: backfire,
       boomerang]
    v 1: come back to the originator of an action with an undesired
         effect; "Your comments may backfire and cause you a lot of
         trouble" [syn: backfire, backlash, recoil]
    2: emit a loud noise as a result of undergoing a backfire; "My
       old car backfires all the time"
    3: set a controlled fire to halt an advancing forest to prairie
       fire