Why is the ninth month called September?
c.1300, demeyne (modern spelling by late 15c.), from Anglo-French demesne, demeine, Old French demaine "land held for a lord's own use," from Latin dominicus "belonging to a master," from dominus "lord." Re-spelled by Anglo-French legal scribes under influence of Old French mesnie "household" (and the concept of a demesne as "land attached to a mansion") and their fondness for inserting -s- before -n-. Essentially the same word as domain.
in English feudal law, that portion of a manor not granted to freehold tenants but either retained by the lord for his own use and occupation or occupied by his villeins or leasehold tenants. When villein tenure developed into the more secure copyhold and leaseholders became protected against premature eviction, the "lord's demesne" came to be restricted and usually denoted the lord's house and the park and surrounding lands