Green’s Dictionary of Slang

1. a young(er) sexual partner.

1912		Sport (Adelaide) 13 July 14/3: They Say [...] That Mum caused jealousy when she left the dance with her Mark Foy. Little Bill has done his chance.
1913		Sport (Adelaide) 26 July 10/2: What was the strong of it, Bertha? Did your ‘Mark Foy’ do you in?
1999		G. Seal Lingo 116: A boy (mark foy in rhyming slang) was a young sexual partner, often shortened to mark.

2. a boy.

1939		Smith’s Wkly (Sydney) 11 Nov. 15/1: To fellow confidence men and the police he is known as ‘the Mark Foy,’ rhyming slang for ‘the boy’; because when he first began to ‘work the tubs’ (card-sharp on passenger liners) he was remark ably youthful in appearance.
1942		Sidney Mirror 14 Oct. in Baker (1945) 269: Blimey no Mark Foy is going to give me a dig in the grave.
1947		(con. 1920s) Truth (Brisbane) 27 July 20/7: [W]hen Coates was caught by a steward, he said. ‘Let me go. I’m- only a “mark foy”’— rhyming slang for ‘boy’.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 36: Mark Foy Boy.
1996		(con. 1964-65) B. Thorpe Sex and Thugs and Rock ’n’ Roll 263: The Crossie boys, or ‘Mark Foys’ as they were known on the street.
2002		Pete’s Aussie Sl. Home Page 🌐 Mark Foy: a boy.