Green’s Dictionary of Slang

1. a mart.

1890

Bulletin (Sydney) 9 Aug. 14/3: [He] filled [the pawnbroker’s tickets] all in with prominent names, and then carefully dropped them one by one in likely places – in a tram-car, one or two in a big club, and others at a jam-tart ‘small and early.’.

2. a sweetheart, a girlfriend.

1885		Observer and Freelance (Wellington) 5 Sept. 4/2: Joe sticks to the Would-be’s-out and out ‘jam tart’ now.
1891		F.W. Carew Autobiog. of a Gipsey 416: My missus – as fly a bewer, she were, as ever chucked a stall, a reg’lar tip-top jamtart and A1 at gladderin’, truckin’, and sallyin’.
1914		Sport (Adelaide) 17 July 4/4: E.G. was out-clessed [sic] by his sparring partner A.T. [...] Herb G. fought five rounds with a jam tart .
1963–74		B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 22: I’ll feature with this jam tart or bust me flamin’ boiler.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.
1977		(con. 1935) A. Campbell Island To Island (1984) 89: Small boys and girls (tarts or ‘jam tarts’, we used to call them).
1980		Barltrop & Wolveridge Muvver Tongue 82: ‘Tart’ has always meant a girl [...] and probably derives from something like ‘jam tart’.

3. the heart, whether anatomically or as a card suit.

1943		M. Harrison Reported Safe Arrival 53: Then ’e shoves the micascope agin me chest, and listens ter me jam-tart.
1979		R. Barker Fletcher’s Book of Rhy. Sl. 39: My jam tart goes with you all.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 95/1: jam tart n. heart.