August ham, n. — Green’s Dictionary of Slang

August ham n.

[the month of the fruit’s ripening + the pinkness of both foodstuffs]

(US black) a watermelon.

1887		Semi-Wkly Fisherman & Farmer (Edenton, NC) 2 Sept. 1/2: Yesterday was September 1. ‘August ham’ — a thing of the past.
1926		Van Vechten Nigger Heaven 285: August ham: watermelon.
1931		N. Van Patten ‘Vocab. of the Amer. Negro’ in AS VII:1 26: August ham. V. n. Facetious for water-melon.