It is a jocular response to somebody impolitely farting in public, along with such responses 'Now you've torn it you'll have to buy it' or 'One more crank at the handle and it will start' and many, many others.

The first _published_ use of the phrase _Katie bar the door_ comes from James Whitcomb Riley's poem 'When Lide Married Him' (1894): When Lide married him - w'y, she had to jes dee-fy The whole poppilation! - But she never bat' an eye! Her parents begged, and threatened - she must give him up - that he Wuz jes "a common drunkard!" - And he wuz, appearantly. Swore they'd chase him off the place Ef he ever showed his face Long after she'd eloped with him and married him fer shore! When Lide married him, it wuz "Katy, bar the door!" That expression must have been in popular use before that time, if the poet's readers were expected to know it. 'Katy (or Katie) bar the door' means take precautions; there's trouble ahead. Canadians regularly use the variant phrase _kitty bar the door_. By extension of the meaning of _Katie bar the door_, _kitty bar the door_ is an ice hockey expression that means to play defensively, play only to prevent a goal [by the opposing team]. Example sentence for "kitty bar the door": In the third period we played kitty bar the door and won 4-2. (The sentence suggests that the winning team had gotten its fourth goal and established a healthy lead before the end of the second period; if they played to protect that lead, they could be expected to win the game.) www.phrases.org.uk/meanings/213750.html