Green’s Dictionary of Slang

1. water used in adulterating milk.

1872

Times 24 Dec. n.p.: Witness generally milked the cows for himself, and then added Simpson at discretion [F&H].

2. adulterated milk.

1872

Standard 25 Dec. n.p.: Simpson is... universally accepted as the title for that combined product of the cow natural and the ‘cow with the iron tail’ [F&H].

3. a milkman.

1902

H. Baumann Londinismen (2nd edn).