Supposed to be a corruption of "Atten-Ash,"--at the ash. Naish, a place near Bristol, England. Naisg, Gaelic, made fast, bound, protected. Probably an old fortress or watch-tower.

Arthur, William (1857) An Etymological Dictionary of Family and Christian Names. New York: Sheldon, Blakeman. Public Domain.

NASH. See prefix. Atle or Atten. A man dwelling by an ash-tree would be called Aten Ash or Atten Ash. See H.R. Ate Nasse, x\d Nasse. On the suppression of the prefix, the N still adhered to the designation of the tree, and the name became Nash. By the same process we got Noakes, Nye, &c. It must not be forgotten, however, that there are localities called Nash, in cos. Salop, Bucks, Monmouth, &c. Yet some of these were originally called Ash. The vill of Esse or Ash, co. Oxford, was corrupted to Nashe, temp. E,ichard II. Dunkin's Oxfordshire i. 177.

Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.