Green’s Dictionary of Slang

1. (Aus.) a free, i.e. non-convict, Australian settler.

1827		P. Cunningham New South Wales II 116: Next, we have the legitimates, or cross-breds, — namely, such as have legal reasons for visiting this colony; and the illegitimates, or such as are free from that stigma.
1945		(ref. to 19C) Baker Aus. Lang. 42: These were the type of people who styled themselves the aristocracy, sterling [...] and, since they had no ‘legal’ reasons for coming to Australia [...] also bore the title illegitimates.

2. counterfeit sovereigns; thus young illegitimate, a counterfeit half-sovereign.

1823

‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc.

3. a poor class of costermonger looked down on by the mainstream costers, selling pea soup, sweetmeats, spice-cakes etc.

1861

(con. 1840s–50s) H. Mayhew London Labour and London Poor I 7/2: The numerous persons who sell only nuts or oranges in the streets [...] or such condiments as peas-soup, sweetmeats, spice-cakes, and the like; those articles not being purchased at the markets. I often heard all such classes called ‘the illegitimates’.

4. used as euph. for bastard n. (1)

1906		Sun. Times (Perth) 21 Jan. 4/7: The sanguinary illegitimate ’ad jinked me fer a tenner.
1916		‘Anzac’ On the Anzac Trail 123: ‘Imshi, you all-fired illegitimates!’.