Green’s Dictionary of Slang

soda n.1

[? faro jargon soda, the top card that is exposed before the start of the game; this card is not counted for betting; see Asbury Sucker’s Progress (1938) 16: ‘An extraordinary number of the terms, technical and otherwise, which were employed by Faro players in the palmy days of the game have passed into the language [...] and are commonly used by millions who never heard of Faro. Here are some of them: [...] Soda — The first card, exposed face up before bets were made. Said to have been a corruption of zodiac’]

(Aus.) something easy to accomplish, a simple task, an easy victim.

[Aus]Baker Popular Dict. Aus. Sl. 69: soda: (of a person) one who is an easy or suitable subject for exploitation. (2) Something easily done.
[Aus]L. Glassop Lucky Palmer 245: It’ll be like soda for a hoop like you.
[Aus]N. Pulliam I Travelled a Lonely Land (1957) 239/1: soda – a cinch.
[Aus]B. Wannan Folklore of the Aus. Pub 34: Old Nick had nine peaceful days; but on the tenth there was Jack bashing his ear on what a soda the job had been.
[Aus]R. McDonald Rough Wallaby 210: A jockey was a ‘fork’ [...] a sure thing a ‘monty’ or ‘soda’.