It's okay. Most native English speakers don't know what 'demesne' means either; it's really obscure.
From dictionary.com
deĀ·mesne
/dɪˈmeɪn, -ˈmin/ Show Spelled[dih-meyn, -meen] Show IPA
noun
1.
possession of land as one's own: land held in demesne.
2.
an estate or part of an estate occupied and controlled by, and worked for the exclusive use of, the owner.
3.
land belonging to and adjoining a manor house; estate.
4.
the dominion or territory of a sovereign or state; domain.
5.
a district; region.