mustard gas 
noun
A colourless oily liquid whose vapour causes severe irritation and blistering of the skin, used in chemical weapons.
- Chemical formula: (ClCH2CH2)2S.
- A chemical weapon is any weapon that uses a manufactured chemical, such as sarin, mustard gas or hydrogen cyanide, to kill or injure.
- All showed traces of blister gases, the army said, a group of chemical compounds which include mustard gas.