Green’s Dictionary of Slang

a person, esp. a motorcyclist, who frequents milk bars.

1951		Age (Melbourne) 13 Sept. 7/2: [T]he chief targets of this road safety campaign are those described by a Christchurch road patrol officer in a court case as ‘milk-bar cowboys’ - those youths with motor cycles ‘who hang around the city milk bars and make nuisances of themselves with noisy exhibitions of their lack of skill’.
1955		Argus (Melbourne) 1 Dec. 3/3: This core could turn the present happy, harmless, idle milk bar ‘cowboys’ into gangs of irresponsible young thugs .
1962		Canberra Times 15 June 2/4: [headline] Milk Bar Cowboy Survey Points to Broken Homes.
1985		J. Sherwood Botanist at Bay 38: I’m sure you don’t talk like a milk-bar cowboy at the commune.
1988		(ref. to 1950s) McGill Dict. of Kiwi Sl. 73/1: milkbar cowboy a postwar bikie, though probably not anymore.
1998		Guardian 5 Sept. 35/2: It’s [i.e. New Zealand] a country where a milkbar cowboy has [...] the advantage over a pillion pussy, though neither breed survived the 1950s.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].