Katie, bar the door!

Katie, bar the door!

"Watch out—there's trouble coming!” King James I, who was set upon in 1437 by unhappy Scots, took refuge in a room whose door had no bar lock. One Catherine Douglas tried to keep the door closed with her arm, but the mob broke through and murdered the king. That the king might have shouted, “Catherine, bar the door,” is not too very different from the once-popular phrase. That the expression was brought to America by Anglo-Scottish settlers is equally likely, and for genera- tions rural folk would acknowledge a difficult situation with a shake of the head and “Katy, bar the door.”
See also: bar