The Collaborative International Dictionary of English v.0.48:

blowback \blowback\ n.
   1. the backward escape of unburned gunpowder after a shot.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

blowback
    n 1: the backward escape of gases and unburned gunpowder after a
         gun is fired [syn: blowback, backfire]
    2: misinformation resulting from the recirculation into the
       source country of disinformation previously planted abroad by
       that country's intelligence service