Early 20th century; earliest use found in The British Medical Journal. Apparently originally from German Yperit from either Yper or Ypern, the names in Dutch regional (West Flanders) and German respectively of Ypres, the town in West Flanders, Belgium, near which mustard gas was first used against Allied troops on 12–13 July 1917 + -it, with remodelling of the ending after -ite.