noun

• (firearms) A type of action where the pressure from the fired cartridge blows a sliding mechanism backward to extract the fired cartridge, chamber another cartridge, and cock the hammer.
  "The Israeli UZI submachine gun, one the most easily recognized examples of a blowback firearm, is well known for its simplicity, ease of manufacture, low maintenance, and utter reliability."
• (slang) The act of shotgunning (inhaling from a pipe etc. and exhaling into another smoker's mouth).
  "Paranoid I was being ripped off, I 'tested' the potency of the skunk on a wasp by trapping it under a glass, and giving it a blowback."
• (rail transport) On a steam locomotive, the reversal of exhaust gases when the regulator is closed without using the blower.
  "The Boulogne stop lasted a few minutes, during which I was surprised to see no use was made of the blower. In this country, if you shut the regulator without the blower on, the odds are there will be a blowback."
• (computing) Synonym of backscatter.