Sesquimustard (military code Q) is the organosulfur compound with the formula (ClCH2CH2SCH2)2.
The compound is a type of mustard gas, a vesicant used as a chemical weapon.
From the chemical perspective, the compound is both a thioether and an alkyl chloride.
Because sesquimustard is a solid at room temperature, it is not as easily deployed as related liquid mustards.
It was only ever deployed as mixtures with the original mustard, with phosgene, or as a solution.