an oily, volatile liquid, (CHClCH)S, used in warfare as a poison gas because of its extremely irritating, blistering, and disabling effects
See mustard gas in American Heritage Dictionary 4
noun
An oily, volatile liquid, (ClCH2CH2)2S, that is corrosive to the skin and mucous membranes and causes severe, sometimes fatal respiratory damage. It was introduced in World War I as a chemical warfare agent.