Green’s Dictionary of Slang

the British army.

1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 134: Kate Karney: The Army.
1943		M. Harrison Reported Safe Arrival 40: You can always do with a grin in the Kate Carney.
1950		P. Hoskins No Hiding Place! 191/1: Kate Karney. Army.
1953		B. Behan Scarperer (1966) 114: Well, they will have the idea you’re on the trot from the Kate Kearney.
1955		B. Hill Boss of Britain’s Underworld 71: I didn’t fancy the Kate Carney, so I put my name down for the Air Force.
1968		P. Hall India Man 73: I’m in the Kate Carney for three meals a day, a free suit, an’ no work.
1971		J. Jones Rhy. Cockney Sl. 19: Kate Karney – Army ‘I’m joining the Kate.’.
1980		Barltrop & Wolveridge Muvver Tongue 11: The rhyming expressions which do get used regularly. [...] ‘when I was in the Kate’ (‘Kate Carney’ = the Army).
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.