Green’s Dictionary of Slang

armshouse n.

(US black/teen) armed gang warfare.

[UK]‘Q’ Deadmeat 203: The buff security guards wore puffer jackets with the logo Armshouse Inc.
[UK]Dizzee Rascal ‘Learn’ [lyrics] Come to the manor get shot, come to the manor get killed. / We call it armshouse, armshouse, go and get your crew. Grime Terminology Guide [Internet] Arms house - Aggression/beef (E.g. I'll bring arms house to your mum's house - I'll bring the beef to your mum's house).