Green’s Dictionary of Slang

battle of Waterloo n.

1. a stew.

1960

J. Franklyn Dict. of Rhy. Sl. 37/1: Battle of WaterlooStew. Early 20C. Cockney usage.

2. a queue.

1946

S. Jackson An Indiscreet Guide to Soho 43: ‘Back to the Battle of Waterloo, lidy,’ he says sharply, and she returns sheepishly to the end of the line!