Green’s Dictionary of Slang

1. an Anglican bishop [the black chimere and white rochet forming his ordinary ceremonial attire].

c.1642		T. Killigrew Parson’s Wedding (1664) III v: The Scrivener’s Wife, that brought the Black-moor from the holy Land, and made him a Brownist, and in pure charity lay with him, and was delivered of a Mag-pie, a Pied prophet [...] for the Mid-wife cry’d out ’twas born a Bishop, with Tippet.
1707		F. Brown Works I 107: Let not those silkworms and magpies have dominion over us [F&H].
1719		in D’Urfey Pills to Purge Melancholy V 285: The Magpyes of late, are confounding the State.

2. (Aus./UK prison) convict clothing (coloured yellow and black); thus the convict who wears it.

1848		Britannia & Trades Advocate (Hobart, Tas.) 13 July 4/2: tho prisoners, who were dressed in partly-coloured clothing, denominated in prisoners’ barrack’s slang, ‘magpie’.
1874		J. Greenwood Wilds of London (1881) 53: There is another class of chained workers called ‘magpies,’ whose dress is black and cinnamon, and these are so punished for assaults on warders and other crimes that may not here be mentioned.
1890		Bulletin (Sydney) 16 Aug. 24/1: Perhaps it is not necessary at this period of colonial history to remark that ‘magpies’ were the worst class of convicts, the term having reference to their ‘yellow and black’ clothing.
1894		‘Price Warung’ Tales of the Early Days 141: Up to this date only the prisoners in the gaol had been supplied with ‘magpie’ — yellow (or brown) and black — clothing.

3. (Aus.) a nickname for a South Australian [the black and white colouring of the bird, the ref. is to the convict clothes worn by early settlers].

1914		Sport (Adelaide) 20 Mar. 5/3: Walter S is [...] taking on the Cross Roads magpie .
1955		N. Pulliam I Travelled a Lonely Land (1957) 235/2: Magpie – person from South Australia.

4. (Aus.) a half-caste.

1982

Australian (Sydney) 30 Aug. 9/7: It’s difficult if you’re a ‘magpie’. You cop it from both sides.

5. (N.Z. prison) an inmate who scrounges off his fellow-prisoners.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 112/1: magpie n. 1 an inmate who begs food and other possessions from his fellow inmates.

6. (N.Z.) a thief [the larcenous propensities of the bird].

2003

McGill Reed Dict. of N.Z. Sl. 131: magpie A thief. Originally WWII Kiwi soldiers referring to somebody taking their tobacco or soap or whatever without asking.