The “Hessians” were German auxiliaries in the 1700’s, contracted for military service by the British government. They took their name from the German state of Hessen-Kassel, where many of them originated. The British hired them for combat in several 18th century conflicts, but they’re mostly associated with combat operations in the American Revolutionary War (1775–1783). About 30,000 Germans fought for the British during that war, making up 25% of the troops the British sent to America. They entered the British service as entire units, fighting under their own flags, commanded by their usual officers, wearing their existing uniforms. The largest contingent came from the state of Hessen, which supplied about 40% of the German troops who fought for the British. This led to the use of the term Hessians to refer to all German troops fighting on the British side, a form of synecdoche. The rest were rented from other German states. Patriots presented them as foreign mercenaries with no stake in America. Many of the men were press-ganged into Hessian service. Deserters were executed or beaten. Hessian prisoners of war were put to work on local farms.
Read more: https://en.m.wikipedia.org/wiki/Hessian_(soldier)