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

Moby Thesaurus II by Grady Ward, 1.0:

116 Moby Thesaurus words for "backfire":
   backlash, backlashing, balefire, bang, beacon, beacon fire, blast,
   blaze, blow out, blow up, blowout, blowup, bonfire, boom,
   boomerang, bounce, bounce back, bound, bound back, burning ghat,
   burst, bust, campfire, cannon, cannon off, carom, cheerful fire,
   combustion, come to grief, comeback, conflagration, contrecoup,
   corposant, counterattack, counterblast, counterblow, counterfire,
   counterinsurgency, countermeasure, counterrevolution, counterstep,
   counterstroke, cozy fire, crackling fire, crematory, death fire,
   defense, detonate, detonation, discharge, explode, explosion,
   fall through, fen fire, fire, fizzle, flame, flare, flash,
   flashing point, flicker, flickering flame, fly back, forest fire,
   fox fire, fulguration, fulminate, fulmination, funeral pyre,
   go off, have repercussions, ignis fatuus, ignition, ingle, kick,
   kick back, kickback, lambent flame, lash back, lay an egg, let off,
   marshfire, miscarry, miss, open fire, prairie fire, pyre,
   raging fire, rebound, rebuff, recalcitrate, recalcitration, recoil,
   repercuss, repercussion, report, repulse, resile, resilience,
   retort, ricochet, sea of flames, set off, sheet of fire, shoot,
   signal beacon, smudge fire, snap back, spring, spring back,
   three-alarm fire, touch off, two-alarm fire, watch fire, wildfire,
   witch fire