Designating a Sabbath observed on the first day of the week (Sunday); designating a Christian denomination, or a member of one, that observes the Sabbath on this day. Usually contrasted with observance of the Sabbath on the seventh day: compare "seventh-day".
In the Religious Society of Friends (Quakers): (a name for) Sunday, as being the first day of the week.
Mid 17th century; earliest use found in George Abbot (1562–1633), archbishop of Canterbury. From first + day.