Green’s Dictionary of Slang

1. (Aus. prison) a prison [the passage of time].

1967

K. Tennant Tell Morning This 439: ‘I suppose you’re about the luckiest bastard in slow [...] How does it feel to have a pardon?’.

2. (Aus. drugs, also slow fluid) a depressant.

1970		K. Mackey Cure xv: Called ‘stoppers’ or ‘slow’ on the scene, it is never pushed. Anyone hung on it goes to the doctor.
1970		K. Mackey Cure 121: S]o take me to the morph where we can discuss this matter in comfort and where I can take a therapeutic smashdown of slow fluid.