Green’s Dictionary of Slang

union jack n.1

[‘Cut it where you will when it’s cooked it’s red, white and blue’ (Binstead, Pitcher in Paradise 1903)]

Argentine frozen beef.

[UK]A. Binstead Pitcher in Paradise 285: Flank of Argentine frozen beef (’union jack’, its patriotic name, is for the reason that cut it where you will when it’s cooked it’s red, white and blue).