Green’s Dictionary of Slang

(W.I.) to gossip, to malign.

1957		F.G. Cassidy ‘Iteration as a Word-forming Device in Jam. Folk Speech’ in AS XXXII:1 51: susu, to whisper.
1983		T. White Catch a Fire 132: He’d cut de firs’ breddah who susu pon ’im, fe certain.