c. 1300, demeine, demeyne (modern spelling by late 15c.), "power; dominion; control, possession," senses now obsolete, 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, master," from domus "house" (from PIE root *dem- "house, household").
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.
Meaning "a manor house and near or adjacent land," kept and occupied by the lord and his family, is from late 14c., hence "any landed estate" (late 14c.). Related: Demesnial.