Green’s Dictionary of Slang

milko n.

also milk-ho, milk-oh, milko man
[SE milk + -o sfx (3)]

1. [20C+] (orig. Aus.) a milkman.

2. [1910s] (Aus.) dawn; the very early morning (i.e. milk delivery time).

3. [1940s] (Aus.) a cow-hand.

4. [1940s] (UK drugs) a mix of heroin and lactose.