An English surname from day as a word for a "day-servant", an archaic term for a day-laborer.Ernest Weekley, The Romance of Words (1927), p. 165. ,or from given names such as Dagr, Daug, Dege, and Dey, cognate with Scandinavian Dag.Susa Young Gates, Surname Book and Racial History (1918) p. 289
An Irish surname anglicised from Ó Deághaidh (“descendant of a person named Good Luck”)