Green’s Dictionary of Slang

bully beef (pickled or tinned beef).

[1838		‘The Stage-Coachman Abroad’ in Bentley’s Misc. IV 605: Then came in a large white dish with little square slices of b’iled beef, as Jack said was ‘bully’ [...] If Dutch courage means a glass of gin, Belgian bully means beef with the strength b’iled out on it].
1908		R. Rankin In Morocco With Gen. D’Amade 39: The tinned corned beef, or ‘bully’ of Tommy Atkins, is dubbed ‘singe’ ("monkey") by the French troops and is not so good as the British article.
1915		Grey Brigade 11 Dec. 3/3: Cold is my dug-out, damp my feet, / Nothing but biscuits and bully to eat.
1924		(con. WWI) A.G. Pretty Gloss. of Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: bully. Abbreviation of Bully Beef. A name given to the preserved meat issued, usually in one pound tins.
1944		L. Glassop We Were the Rats x: It’s practically all bully beef and biscuits here, George. No matter how cleverly the cook disguises the old bully, we always see through it.
1959		(con. 1940s) G. Slatter Gun in My Hand 184: Don’t choke on the bully we said.
1977		(con. 1941) R. Beilby Gunner 16: Several tins of precious bully.
1981		J. Henderson Exiles of Asbestos Cottage 98: Sometimes he’d take a dozen tins of bully.