Green’s Dictionary of Slang

bally n.2

also bali

(UK black) a balaclava; thus bally / bally’d up, to wear / wearing a balaclava.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 13/1: ballie up to put on a balaclava with criminal intent, in order to perform a burglary, robbery, daylight murder, rape, or any other crime that requires the wearing of a balaclava to ensure the perpetrator’s anonymity:.
www.reddit.com/r/hiphopheads Grime Terminology Guide 🌐 Bally - Balaclava.
Unknown T ‘Homerton B’ 🎵 Opp block, bally on me / Panic and dash / Dem boy run and retreat.
[UK]T. Thorne (ed.) ‘Drill Slang Glossary’ at Forensic Linguistic Databank 🌐 Bally, Bali – balaclava.
[UK]G. Krauze What They Was 201: Bunny runs up he yard [...] bally’d up, holding a Glock 9.
Billy Billions ‘Tizzy’ 🎵 No gloves, no bally, but I still came / And man can’t tell me ’bout gunplay.

In phrases

ballie someone up (v.)

(N.Z. prison) to cover a victim’s head with a blanket or pillowcase prior to administering an anonymous beating.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 13/1: ballie (someone) up to put a blanket or pillowcase over a victim’s head so as to beat him up without being identified.