Green’s Dictionary of Slang

casaba n.2

[SE cassava, ‘A plant, called also by its Brazilian name Manioc [...] two varieties (or species) of which are extensively cultivated in the West Indies and tropical America, as also in Africa, for their fleshy tuberous roots, which yield the greatest portion of the daily food of the natives of tropical America’ (OED)]

(US) the head.

[US]B. Schulberg On the Waterfront (1964) 192: What was he doing — getting soft in the casaba.