Louis (along with Clovis, the version used in French to refer to Franks in the time of the early medieval Merovingian dynasty) is descended from the Germanic name Hlodowig, meaning something like "fame in battle," via the Latinized form Chlodovechus, which has the regular Latin ending "-us" added. The languages that became French still retained more features from the Latin system of word endings than Modern French does, particularly in proper names, so "-es" was a regular noun ending derived from "-us." The "-es" ending survives today in (perhaps slightly old-fashioned) French men's names: see Jules, from Julius; Jacques, from Iacobus, the Latin rendering of ancient Hebrew Ya'akob; Charles, the Gallicized form of Germanic Carl; Georges; etc. The sound changes that made those Ss (Ses?) silent happened later, approximately between the years 1300 and 1700, but by the end of that time, the institutions that executed increasingly centralized and far-reaching royal authority had seized on the idea of a national official language, not Latin, but with standard spelling that was supposed to derive its legitimacy from showing the language's Latin roots. Officially founded in 1635 by order of King Louis XIII, the Académie française based a lot of the rules it came up with on made-up etymology, like spelling "savoir" ("to know") as "sçavoir" to suggest the Latin word "scire," which is not actually the direct ancestor of "savoir." Some of the Académie's prescribed spellings have since been decertified, so to speak, but many of them stayed in effect if they had a real historical basis or followed a more generally applicable rule. The S in Louis happens to have both qualifications.