poison gas

poison gas (countable and uncountable, plural poison gases or poison gasses)

1. A poisonous gas.
   1. (military) Any poisonous gas used as a chemical weapon.
      • 1939 November, Charles E. Lee, “Railways and the War — I”, in Railway Magazine, page 315:

        [...] approximately 23,000 members of the L.M.S.R. staff were trained in methods approved by the Home Office for dealing with the consequences of an air raid, including the effects of poison gas and incendiary bombs.