Green’s Dictionary of Slang

(W.I.) a greedy, omnivorous person; thus also as adj.

1960–82		L. Bennett Auntie Roachy Sey (2003) 34: An de waistcoat grampa dead lef / Nyamy-nyamy up and tear!
1966		(ref. to 1940s) L. Bennett Jamaica Labrish 223: nyamy-nyamy. given to eating, gluttonous, disfigured.
1995		Francis-Jackson Official Dancehall Dict. 37: Nyamy-nyamy gluttonous: u. yuh too nyamy-nyamy.