Wiktionary
methyldichloroarsine
n. A highly toxic organoarsenic compound, CH3AsCl2, that has been used in chemical warfare.
Wikipedia
Methyldichloroarsine
Methyldichloroarsine, sometimes abbreviated "MD", is an organoarsenic compound with the formula CHAsCl. This colourless volatile liquid is a highly toxic vesicant that has been used in chemical warfare.