Green’s Dictionary of Slang

(Aus.) in two-up, the ‘tail’ of a coin; thus as v., to spin the coins so that they come up tails.

1919		W.H. Downing Digger Dialects 33: mick (n.) — (1) The Queen’s head on a coin (e.g. ‘Micks are right’ when two heads have turned up in a game of two-up).
1924		(con. WWI) A.G. Pretty Gloss. Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: micks. The tails of the pennies used in a game of ‘Two-Up.’.
1941		Baker Popular Dict. Aus. Sl. 46: Mick, the ‘head’ of a penny.
1953		T.A.G. Hungerford Riverslake 126: I got ten bob to say he tails ’em – ten bob the micks!
1999		G. Seal Lingo 150: Bets are won or lost on whether the result is two heads (skulls, nuts, neds); two tails (two micks), or one of each (ones).