I was going to use the phrase "Katy bar the door" in an article the other day and wanted to check that I actually understood the meaning...I was thinking of it along the lines of "hold back the floodgates."
The Phrase Finder defines it more along the lines of "there's trouble ahead," and tells an intriguing story that may or may not be true about its origins. Apparently Katy couldn't really bar the door, and got a broken arm (ow!) for her trouble.
Excerpt: "Another suggestion is that the phrase originates with the story of Catherine Douglas and her attempt to save the Scottish King James I. He was attacked by discontented subjects in Perth in 1437. The room he was in had a door with a missing locking bar. The story goes that Catherine Douglas tries to save him by barring the door with her arm. Her arm was broken and the mob murdered the King. The 'lass that barred the door' - Catherine Douglas, was henceforth known as Catherine Barlass."