Green’s Dictionary of Slang

1. an attractive (young) woman, thus as term of affectionate address.

1842		Flash (NY) 2 Oct. n.p.: Hudson Wants to Know [...] Whether W.H. has picked his plums in Chapel street yet.
1898		F.W. Hume Hagar of the Pawn-Shop 161: She’s a plum, ain’t she?
1905		‘The Overlander’ in ‘Banjo’ Paterson Old Bush Songs 120: Each has a sweetheart there, / Dressed out in all her grandeur— [...] ‘She’s a plum,’ says the overlander.
1908		Bulletin (Sydney) 3 Sept. 39/2: ‘I have made an appointment with two peaches for both of us at half-past two.’ [...] ‘Peaches – who are they?’ / ‘Some girls. They’re real plums – Up to snuff, too – won’t go out without one another – so you’ve got to come along and take one off my hands.’.
1914		Sport (Adelaide) 20 Mar. 5/4: Long Percy M says he has got the best tart of any one about. Is she a peach or a plum, Perce.
1922		T.E. Spencer ‘Liza’ in Budgeree Ballads 87: You’re a daisy! You’re a plum! You’re— well, you’re Liza!
1983		J. Byrell (con. 1959) Up the Cross 16: ‘Well now, my plum I [...] see you’ve finished your drink’.

2. a good horse.

1890

‘Rolf Boldrewood’ Colonial Reformer III 48: He’s a plum, that’s what he is! [...] He’s the best horse in these parts by long chalks.