From the etymology, I would assume it means just the native sitting in/on the national land.
Means: no foreign mercenaries were accepted for this new corps,
Land-wehr literally means land defense.
Land-s-er comes from, I believe Land + the euphonic sound "s" + the suffix "er" which here means a man: so the original meaning was; (fellow) countryman, later: a German soldier.
"Landser" appears in the German language as late as the 20th century, I am very sure it was not in use during the Napoleoinc wars.