Definition of backfires:
(v) :
(idiomatic, transitive) To fail in a manner that brings down further misfortune.
(v) :
(of a gun, cannon, Bunsen burner, etc., intransitive) To fire in the opposite direction, for example due to an obstruction in the barrel.
(v) :
(of a reciprocating engine, intransitive) To experience a premature ignition of fuel or an ignition of exhaust gases, making a popping sound.
(n) :
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 the wrong direction.
(n) :
An explosion in the exhaust passages of an internal combustion engine.
(v) :
(firefighting) To set a back fire (small, controlled fire to limit spread of a larger fire).