Green’s Dictionary of Slang

(Aus.) a young woman; cite 1943 suggests her physique.

1934		N. Lindsay Cautious Amorist 111: ‘[T]his little wench is with him at the time, a little dressed-up slim-dilly’.
1943		Dly Teleg. (Sydney) 27 Mar. 12/4: Why do young men of today prefer slim girls? These slim-dillies are the ones who give men white feathers. I prefer plump lasses.
1955		N. Pulliam I Travelled a Lonely Land (1957) 239/1: slim-dally [sic] – a girl or young woman.