Green’s Dictionary of Slang

1. (orig. milit.) bread or ship’s biscuits soaked in tea or coffee.

1890–1904		Farmer & Henley Sl. and Its Analogues.
1911		F. Martyn Life in the Legion 61: [A]bout half-a-pint, of sweetened black coffee is brought to the legionary’s bedside before he gets up in the morning, and with this some men take bread saved from the previous day’s allowance; generally breaking it up and putting it into the coffee—making ‘slingers’ of it, as the British soldier used to call the operation .
1915		Lichfield Mercury 4 May 5/2: Slingers—A meal of bread and tea.
1916		John Bull 8 Jan. 7/2: Before the war [...] it was possible to procure ‘a little something for breakfast.’ This proved too great a burden for the Master Cook and Q.M.S. and so it has been stopped and, say the [Chelsea] Pensioners [...] in a sentence striking but mysterious — ‘Now we have to fall back on Slingers!’.
1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 261: Slingers: Tea or coffee with bread soaked in it.

2. sausages.

1937		J. Curtis You’re in the Racket, Too 12: ‘Them slingers is done. You’ll be burning the cowsons if you don’t watch out.’ A little fat man transferred the sausages from the pan to the plate.
1945		Lethbridge (Alberta) Herald 17 Nov. 10/3: First Soldier (surveying his dinner) ‘Slingers and Gippo again!’ [...] Slingers and Gippo – sausages and gravy.