As you can see from the map below, that serene spot at the western end of the Ile de la Cité was once a separate island, called Ile des Juifs (Island of the Jews, or Jew Island) because some Jews were burned at the stake there some time during the Middle Ages.
More famously, Jew Is. was the spot where the leading Knights Templar were slowly barbequed in 1307 (see pic). You can see how the painter of the execution depicted Jew Is.
Later on, when the Pont Neuf was built, Jew Is. was appended to the Ile de la Cité.