Green’s Dictionary of Slang

(US) used as a warning, to indicate impending danger; cite 1878 differs slightly, suggesting inclusiness rather than exclusiveness.

[1872		Louisiana Democrat (Alexandria, LA) 2 Oct. 2/3: The Custom House Packet, with the Custom House colored band, U.S. Marshal Packard, in command, with the old flag triumphantly kissing the breeze of old Red, the band playing ‘Katie, Bar The Door,’ and with waving rags touched the wharf and proceeded to land her precious cargo].
1878		Sedalia Wkly Bazoo 26 Mar. 3/7: This winsome maid had lovers many, / Whose love she did implore, / [...] / But when the soldier he came in, / It was ‘Katie bar the door’.
1879		Times-Democrat (Lima, OH) 30 Oct. 3/4: I saw it was ‘Katy bar the door’ with me unless I skipped, and I lost no time in skipping.
1888		Wichita Eagle (KS) 30 Oct. 3/3: He saw that ef I came across an Ingin hit’d be ‘Katie, bar the door’ with that savage.
1890		Ft Worth Gaz. (TX) 12 Sept. 5/3: Some of the old time workers [...] must buckle on the harness and push this matter, or the time will soon come when it will be ‘Katy, bar the door’.
1902		Paducah Sun (KY) 18 Jan. 5/5: If a cold wave should ‘hit us’ with the fast declining rivers it would be ‘Katy, bar the door’ with steam-boat men .
1911		Wash. Times (DC) 19 May 12/2: But the naps [a baseball team] — wow, Katie bar the door. They grabbed seven runs.
1918		Eve. World (NY) 21 June 14/6: From there home it was a case of ‘Katie, bar the door’.
1922		Eve. World (NY) 7 July 14/6: The indians were two runs in the lead and it looked like ‘Katie, bar the door’.
1934		Z.N. Hurston Jonah’s Gourd Vine (1995) 155: When Ah colleck dem few pennies Ah got owing tuh me, iss good bye Katy, bar de door.
1944		D. Runyon Runyon à la Carte 35: It is Katie-bar-the-door for the water front and the subs lying along the quays.
1948		Dan Burley ‘Back Door Stuff’ 6 Nov. [synd. col.] From then on in, it was Kitty Bar the Door.
1955		‘Blackie’ Audett Rap Sheet 36: She hauled them out and opened them up, right there in the alley, and you can just imagine! Holy mackerel and Katy bar the door!
1971		T. Thackrey Thief 121: It’s a felony like leaving the scene of an accident or killing some dude when driving drunk – it’s Katie-bar-the-gate.
1983		Hill & Smith Outdoor Yarns 77: Then, all normal functions ceased and it was Katie-bar-the-door.
1994		R. Atkinson Crusade 82: If they’ve been hit with chemicals, Katie bar the door because they’re going to do something.
2002		D.H. Sterry Chicken (2003) 73: Then Kristy wriggles [...] Well, that was that, and Katie, bar the door.
2004		A. Kleinzahler Cutty, One Rock (2005) 76: They decided they had been ‘unfairly discriminated against on the basis of color.’ Whoa, Katie bar the door!