Explanation: Hesse is a state in modern Germany. It used to be several, smaller independent polities. Outside of the American Revolution, I'm not aware of those states having made particularly noteworthy names of themselves.
However, during the American Revolutionary War, the British Empire hired some 30,000 soldiers from these princely German states - nearly as many German mercenaries as British soldiers served in the Loyalist ranks. While not all of those mercenaries were actually Hessian, enough were that the name caught as a slang term for German mercenaries in the Revolutionary War. More numerous and 'foreign' than the British professional troops ('redcoats'), Hessian soldiers acquired a fearsome reputation during the war. Considering that they were largely trained and professional soldiers, and many Americans (Patriot or Loyalist) fighting in the war were militia, not unfairly.
While the Hessians were good soldiers, being sent across an ocean to fight another country's battles so your ruler can get a little money can rankle a bit. Especially bad if you're a conscript to begin with. Many Hessians who surrendered or were captured chose instead to settle in the USA, which already had several large German-American communities, rather than return to their countries of origin in Germany after the war.
