The name is based on the verb croquer ("to crunch") and the word monsieur ("mister")—the reason behind the combination of the two words is unclear—and is colloquially shortened to croque. While the origins of the Croque-monsieur are unknown, there are many speculations on how it was first originated. One such story is that a long time ago there were French workers who would take their sandwiches to work with them. Some would take ham and cheese, and since they did not have coolers or refrigerators, they would leave their lunches by the radiators, and the cheese would melt. The Croque-monsieur's first recorded appearance on a Parisian café menu was in 1910. Its earliest published use has been traced back to volume 2 of Proust's À la recherche du temps perdu (1918).
A ham and cheese sandwich snack, very similar to the Croque-monsieur, is called a Tosti in the Netherlands.
A croque-monsieur served with a fried egg or poached egg on top is known as a croque-madame (or in parts of Normandy a croque-cheval). Many dictionaries attribute the name to the egg resembling an old fashioned woman's hat, although, it is really meant to resemble a womans breast. According to the Petit Robert dictionary, the name dates to around 1960. The name croque-mademoiselle is associated with many different sandwiches, from diet recipes to desserts.
Versions of the sandwich with substitutions or additional ingredients are given names modelled on the original croque-monsieur, for example: