Green’s Dictionary of Slang

jack n.24

(N.Z. drugs/prison) a milligram weight, usu. referring to a pill.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 94/1: jack n. 1 one milligram (esp. indicates the strength of drugs in tablet form): ‘The misties are one dollar a jack.