Green’s Dictionary of Slang

(W.I. Rasta) marijuana, often particularly potent strains.

1995		Francis-Jackson Official Dancehall Dict. 26: Ishen marijuana of a potent form grown in the hilly areas; money.
2002		Christafari 🌐 Marijuana is called many different names: Ganja, Ishence, Sleng teng, Cashi [etc.].
2004		A. Mansbach ‘Crown Heist’ in Brooklyn Noir 133: That’s weed, Jumpshot. Collie. Ishen. Ganja. Sensi. Goat shit.