Green’s Dictionary of Slang

milko n.

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

1. (orig. Aus.) a milkman.

[[Aus]Bell’s Life in Sydney 29 Aug. 2/5: The defendant came before him and challenged Milky to a shindy].
[[Aus]W.A. Sun. Times (Perth) 2 Jan. 3/6: One of the officer’s chargers had served its apprenticeship in a milkman’s cart, and as its daily driver saw it pass, he called out, ‘Milk, oh!’].
[Aus]Sydney Sportsman (Surry Hills, NSW) 27 Feb. 2/6: He is also well known in the ‘milk ho’ circles round Pyrmont.
[Aus]W.A. Sun. Times (Perth) 19 Jan. 1/1: The official in question has no less than seven Government billets, not to mention his ‘milk O!’ occupation.
[Aus]Sun. Times (Perth) 7 Feb. 1/1: Milk-O men owning dry cows literally starve them until springing time.
[Aus]Bulletin (Sydney) 6 July 12/1: [T]he milkman called, whereupon the marauder appeared with a jug and said, ‘A pint and a half, please.’ Thinking it over later ‘milk-o’ decided it was curious and spoke to the Law.
[Aus]Bulletin (Sydney) 1 Sept. 10/3: [pic. caption] Milko. ‘Pint, sir?’ [Ibid.] 8 Dec. 10/4: [T]he two-gallon licence holders at Powlett (Vic.) vend their wares in handy spring carts, just as the common or noisy Milk-oh! does. And the billy is stuck out just as the milk jug is.
[NZ]Truth (Wellington) 22 May 7: James Haslam [...] is a milk-o. He was carting round the early morning chalk and water to ther ‘torfs’.
[UK]Breton & Bevir Adventures of Mrs. May 54: I’ve known ’em come and knock as though they was the postman [...] or call out ‘milko’ at the back door.
[Aus]Mirror (Perth) 23 Feb. 20/7: Milko! He arrived early in the morning.
[Aus]Mirror (Perth) 14 Feb. 1/4: Milko is K.O’D. A milkman was attacked, smashed over the head and left unconscious.
[Aus]Mirror (Perth) 30 Mar. 6: [headline] £200 Damages for ‘Milko’ Who Lost Wife to ‘Trammie’.
[Aus]Aus. Women’s Wkly 25 Feb. 15/2: A milko received a note saying: Leave two pints ordinary milk.
[Aus]D. Ireland Glass Canoe (1982) 154: They’d been there so long, the milko delivered milk to the car.
[Aus]N. Keesing Lily on the Dustbin 118: The ‘milko’ poured the daily order into the household billy.
[Aus]J. Birmingham Tasmanian Babes Fiasco (1998) 3: He was a milko with a van and a milk run.

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

[Aus]Bulletin (Sydney) 12 Dec. 38/4: Well, I never once got home between midnight and milk-o without havin’ that flamin’ torch flashed in my face.

3. (Aus.) a cow-hand.

[Aus]F. Clune Try Nothing Twice 111: What with watering the milk and letting calves strip the cows, I was candidly a failure as a milk-oh [AND].

4. (UK drugs) a mix of heroin and lactose.

[UK]V. Davis Phenomena in Crime 99: In conclusion, I would introduce the authorities to the latest export of the dope ‘rings’. It is called ‘Milko’, and is a concoction of heroin and lactose, a milk by-product. It costs one shilling and twopence a gramme and it has been introduced by the racketeers as a substitute for other drugs the last Great War has restricted.