Green’s Dictionary of Slang

yam n.

[West African words such as Hausa nama, flesh, meat; Swahili nyama, meat; Fulah nyama, to eat; all these in turn based on SE yam, a variety of edible tuber]

1. food; ‘this word is used by the lowest class all over the world; by the Wapping sailor, West India negro, or Chinese coolie’ (Hotten, 1867).

[WI]P. Marsden Account Island Jamaica 49: They are as big as our magpies, and have bille like parrots. The negroes say, the black parroquets are good for yam, i.e. good to eat .
[UK]Farmer & Henley Sl. and Its Analogues.

2. (US black) a West Indian.

[US]M. Agar Ripping and Running 165: Yam – Negro.

3. (US) a usu. derog. description of a man.

A. Baer Putting ’Em Over 30 May [synd. col.] There ain’t much left of the bashful yam’s speech except the commas and the period.
[US]R.E. Howard ‘Bull Dog Breed’ Fight Stories Feb. [Internet] Steve, this yam is challengin’ you to a duel.

4. (US drugs) a balloon filled with a powdered narcotic, usu. heroin.

genius.com [Internet] Big butts on healthy ladies and balloons filled with drugs (particularly heroin) are also colloquially known as ‘yams’.