Green’s Dictionary of Slang

(N.Z. prison) of marijauna, to be rendered second-rate.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 62/1: dross (also dross up) v. to ruin, to make poor (applied specifically to marijuana): ‘When it arrived, the marijuana was all drossed up, just bits of leaf and head.