Considering the date of the map, it's almost certainly referring to leagues. My best guess is that it's two abbreviations, something like Lac.Lies. Where the lies would be an abbrev for lieues, and the Lac would be abbreviation for what kind of lieues they are. Lieues could vary in length according to what city or province you were in. France didn't go decimal until around 1800. Hopefully that helps...

I could be completely wrong of course, 18th century french isn't my strong point