Green’s Dictionary of Slang

armoured cow n.

1. tinned milk.

[US]S.F. Chronicle 1 June H5/7: The draftees assigned to Camp Claiborne, in Louisiana, got out a glossary of slang terms to describe everyday things in army life [...] they call canned milk an ‘armored cow,’ and the white fish a ‘sewer trout’.
[US]J.W. Bishop ‘American Army Speech’ in AS XXI:3 Oct. 196: If a soldier in the mess hall wants the canned milk, he is presumed to say, ‘Pass the armored cow, please.’.
(con. WWII) V. Renouf Forfeit to War 147: Just before it was time to go ashore, Stripey went below to divide up the ‘armoured cow’ (as the sailors called tinned milk).

2. canned corned beef.

[UK]I. & P. Opie Lore and Lang. of Schoolchildren (1977) 183: ‘Armoured cow’ proclaims that tinned corned beef is on the menu.