Today it means: A chestnut-based frozen dessert, generally agreed to contain pureed chestnuts, mixed with cream or egg custard, flavoured with dried fruit, including currants, raisins and dried cherries, candied peel and brandy or other spirits. Full definition →
Named after the Russian diplomat Karl Nesselrode (1780–1862). Generally attributed to Chef Mony (or Muny, Munie, Monni or Mouy), who invented the basic idea sometime before 1840 while working for him.
Etymology from Wiktionary, CC BY-SA 4.0.