Catherine Douglas, later Barlass, was a 15th-century Scottish noblewoman who tried to prevent the assassination of King James I of Scotland on 20 February 1437. She was a lady-in-waiting to the Queen of Scots, Joan Beaufort.
James fled into a sewer tunnel as the queen and her ladies quickly replaced the floorboards to hide his location.[3] Catherine sprang to the door and placed her arm through the staples to bar the assassins' entrance.[4] However, they forced the door open anyway, breaking Catherine's arm, and discovered and killed the King. From that point on, according to the story, Catherine took the surname of "Barlass".
The idiomatic phrase "Katie, bar the door!" (a warning of the approaching trouble) may have its origins in the story of Catherine Douglas.[5]Dante Gabriel Rossetti recounted the story of Catherine Douglas in verse in 1881, under the title "The King's Tragedy". This poem contains the line "Catherine, keep the door!"