Green’s Dictionary of Slang

1. (Aus.) abbr. magpie.

1901		Bulletin (Sydney) 5 Oct. 17/1: Re the black-backed Monaro magpies [...]. Has ‘Gumleaf’ or any other observer ever seen two ‘maggies’ marked alike?
1934		Bulletin (Sydney) 19 Sept. 21/2: Out maggie went backwards through the door, arguing every inch of the ground.
1941		Baker Popular Dict. Aus. Sl.
1982		N. Keesing Lily on the Dustbin 78: Kids with red or fair hair seem to be more at risk from ‘dive-bombing maggies’ than those with darker colouring.

2. (US Und.) an automatic pistol [its magazine of bullets].

1914

Wash. Post 11 Nov. Miscellany 3/6: An automatic is generally designated as a ‘maggie.’.