Green’s Dictionary of Slang

(UK black/gang) a gun.

2018		Loski ‘Cool Kid’ 🎵 I need more gyal to put my mashes with.
2018		Loski ‘Money & Beef’ 🎵 Baby if you love me then you’ll hold on to my mash'.
2020		G. Krauze What They Was 56: I told him the mash was rusty.