Well son of a gun, Don knows what he's talking about. I always thought it was "kitty-bar the door".
Meaning
'Katy (or Katie) bar the door' means take precautions; there's trouble ahead.
Origin
This phrase is little used outside the USA. It may or may not have originated there. The first known use in print of Katy bar the door with the meaning of 'trouble is in store' is in James Whitcomb Riley's poem When Lide Married Him, 1894:
http://www.phrases.org.uk/meanings/213750.html