What is the source for the word "raguser", a synonym for betrayal in France?

Auguste Frédéric Louis Viesse de Marmont was a late 18th and early 19th century French general and nobleman who was awarded the title of "Duc de Raguse" (in English, the "Duke of Ragusa" - Ragusa being the former name of Dubrovnik in Croatia). Halfway through the seven year Peninsular War fought in the Iberian Peninsula, he took command of the French army, but lost decisively at the Battle of Salamanca as France lost the whole war. His surrender to the opposing forces and subsequent loyalty to them was considered a great betrayal to Napoleon, and the verb "raguser", derived from his title, became a household word in France.

https://www.napoleon-series.org/research/biographies/marshals/c_marmont.html
https://en.wikipedia.org/wiki/Auguste_de_Marmont
https://en.wikipedia.org/wiki/Peninsular_War

Rather like "Quisling" in some other languages":

https://en.wikipedia.org/wiki/Quisling