Transferred use of the surname, originally a local name for someone who lived in a dale or valley. It is now fairly commonly used as a given name, along with other monosyllabic surnames of topographical origin (see for example Dell and Hale).
Irish (especially Ulster): 1. Anglicized form of a rare Gaelic name, Mac Nuadhat, a patronymic from the personal name Nuadha, the name of an ancient Celtic god. 2. (originally Scottish) reduced form of McNaughton.