Kathleen is of Irish origin, derived from the Gaelic 'Caitlín', which means 'pure' or 'clear'. It was popularized by Saint Kathleen of Kildare, a 6th-century nun and abbess known for her great learning and wisdom. The name gained further popularity through the works of Sir Walter Scott in his poem 'The Lady of the Lake', where he used the name Kathleen as one of his characters.
Kathlean was a popular name for newborn girls from the early 20th century until the late 1970s, with peaks in popularity between the 1940s and 1950s.