Scarlet woman

A scarlet woman is a sexually promiscuous harlot and the expression has been in use since the late 16th century. The source is the New Testament where “the mother of harlots” (Revelation 17:5) was “arrayed in purple and scarlet colour” (Revelation 17: 4). In the late Middle Ages, according to the OED, ‘scarlet woman/lady or whore’ was used as a nickname for the Church of Rome because of the promiscuous conduct of some of its Cardinals who wore scarlet robes.