Source : Webster's Revised Unabridged Dictionary (1913)

Back fire \Back fire\
   (a) 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.
   (b) 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, an explosion in the
       exhaust passages of such ah engine.