Green’s Dictionary of Slang

(orig. US campus) large female breasts.

1967–8		Baker et al. CUSS.
1989		P. Munro Sl. U.
1993		M.B. ‘Chopper’ Read How to Shoot Friends 111: By this time she was a big-built woman with a large set of Polish watermelons.
1999		P. Temple Black Tide (2012) [ebook] [She] appears to have shoplifted watermelons and put them down the front of her dress.