Hessian (n.)
"resident of the former Landgraviate of Hessen-Kassel," western Germany; its soldiers being hired out by the ruler to fight for other countries, especially the British during the American Revolution, the name Hessians by 1835 in U.S. became synonymous (unjustly) with "mercenaries." Hessian fly (Cecidomyia destructor) was a destructive parasite the ravaged U.S. crops late 18c., so named 1787 in erroneous belief that it was carried into America by the Hessians.

The place name is from Latin Hassi/Hatti/Chatti, the Latinized form of the name of the Germanic people the Romans met in northern Germany (Greek Khattoi). The meaning of the name is unknown. Part of Arminius's coalition at the Battle of Teutoburger Wald (9 C.E.), they later merged with the Franks. They are mentioned in Beowulf as the Hetwaras. The state was annexed to Prussia in 1866 and is not to be confused with the Grand Duchy of Hesse-Darmstadt.