No, it's not a word you're going to run across much nowadays, but if you happen to be working your way through Gibbon's The History of the Decline and Fall of the Roman Empire, you may be surprised at how much it comes up. I have a good enough grasp of the sense of it (in context) to know it means something like property or grounds or land, but I thought I should probably get a little clearer idea of it, because I suspect it's not the last time I'm going to run across it while reading this multivolume work.
|Castletown House and demesne, County Kildare, Ireland|
If you look up a word's definition on Google, you can also find a chart on its historical and current usage, and demesne's drop has been pretty steep over the years. (Gibbon published his work from 1776 to 1789, so the chart doesn't even quite cover his period.) But the word does show up in other older works, so it's good to have a handle on it.
"Demesne", which is pronounced di-MAYN or di-MEEN, comes to English by the very standard route of Anglo-French (demesne or demeine) from Old French demaine, meaning 'land held for a lord's own use', which eventually leads us back to the Latin dominicus, "belonging to a lord or master", which stems from dominus "master", and domus, "house." I liked this uncharacteristically humorous explanation from the Online Etymology Dictionary, which explains that Anglo-French legal scribes changed the spelling under the influence of the Old French word mesnie, meaning household, "and their fondness for inserting -s- before -n-."
|Medieval Manor--mustard color for demesne|
"Demesne's" meaning has grown to be pretty broad. It still retains the sense of being an estate or part of an estate that is occupied and held by and worked for the exclusive use of its owner. It can simply be the land adjoining a manor house. But it can also mean the dominion or territory of a sovereign or state, or it can just mean a district or region.
Now, the funny thing is that the Online Etymology Dictionary ends up by telling us that "essentially", it's the same word as "domain." That's because after "demesne" made its way into England around 1300, the term came in again through Scotland, from Middle French domaine, though tracing back to the same Latin roots.
This got me to wondering what the French was for "domain name". Could it be nom de demesne? Unfortunately, though reasonably, it is simply 'nom de domaine'. At least they got back their beloved final -e-.