Green’s Dictionary of Slang

1. (orig. Aus.) rum.

1905		Duke Tritton’s Letter n.p.: I can go into the Rubbity Dub and have a lemonade, breasting the Near And Far with booze hounds drinking Tom Thumb, Young And Frisky, Oh My Dear, or Huckleberry Finn, and no one ever laughs at me or calls me sissy because I am drinking lolly water.
1919		Sun. Times (Perth) 9 Mar. n.p.: Gin is known as mother’s ruin; rum as Tom Thumb.
1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 287: Tom Thumb: Rum.
1934		Advertiser (Adelaide) 20 Jan. 11/7: ‘Tom Thumb’ rum.
1940		Barrier Miner (Broken Hill, NSW) 30 Mar. 5/2: The digger who went along to order some rations from the British Tommy [...] ’Battalion Halt, Wasp and Bee, Soap and Suds, Tom Thumb’.
1941		(ref. to 1917) Argus (Melbourne) 15 Feb. 1s/4: [photo caption] ‘Hopping in for their issue of Tom Thumb’ in a ruined French town in 1917 — ‘Tom Thumb’ — rum.
1953		J. Franklyn Cockney 294: In order to keep the love and kisses (missis) quiet he’d had to buy her a bottle of Tom Thumb (rum).
1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xli 4/4: tom thumb: Rum.
1969		S.T. Kendall Up the Frog.
1971		J. Jones Rhy. Cockney Sl.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 47: Tom Thumb Rum.
2002		Pete’s Aussie Sl. Home Page 🌐 Tom Thumb (1): rum.

2. the buttocks [bum n.¹ (1)].

1969		S.T. Kendall Up the Frog.
1971		Daily Tel. 17 Sept. n.p.: Tom Thumb = ‘the rear as opposed to the front projections.’.

3. (Aus.) inside information [drum n.⁶ ].

1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xli 4/4: tom thumb: [...] rhyming slang for the drum, give the strong tip, drum a person up, give him the mail.
1999		G. Seal Lingo 90: Others of this type include tom thumb for drum, as in it’s the tom thumb, meaning the truth, or useful information of some kind.
2000		R.G. Barrett Leaving Bondi (2013) [ebook] ‘I need the Tom Thumb on that cook. [...] What do you know about him?’.