Green’s Dictionary of Slang

(US black/teen) armed gang warfare.

1997		‘Q’ Deadmeat 203: The buff security guards wore puffer jackets with the logo Armshouse Inc.
2004		Dizzee Rascal ‘Learn’ 🎵 Come to the manor get shot, come to the manor get killed. / We call it armshouse, armshouse, go and get your crew.
2015		www.reddit.com/r/hiphopheads Grime Terminology Guide 🌐 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).