Green’s Dictionary of Slang

(N.Z. prison) a morphine sulpate tablet.

1999		D. Looser ‘Boob Jargon’ in NZEJ 13 33: mistie n. Morohine SulDhate Tablet (MST).
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 117/2: mistie n. a morphine sulphate tablet.