[OED:] The Anglo-Norman spelling demesne of the law-books, and 17th cent. legal antiquaries, was partly merely graphic (the quiescence of original s before a consonant leading to the insertion of a non-etymological s to indicate a long vowel), as in
mesne = Old French meien , meen , mean , modern French moyen ;
partly perhaps influenced by association with mesne itself, in ‘mesne lord’,
or with mesnie < mansionāta house, household establishment.
Seeing only one s in demesne, I am confused by the OED:
Where is the original s? Where is the non-etymological (so epenthetic) s? Etymonline states less.