Green’s Dictionary of Slang

(Aus. prison) to arm oneself.

1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Arm-up. To arm oneself with a weapon such as an iron bar or knife.
2005		M.B. ‘Chopper’ Read Chopper 4 220: The crims armed up, so the cops armed up.