Category:Cockney rhyming slang A gun or in particular an old or wornone.
Origin
From Middle English rusty, from Old English rūstiġ, from Proto-Germanic *rustagaz, equivalent to rust + -y. Cognate with Saterland Frisian rusterch, West Frisian rustich, roastich, Dutch roestig, German Low German rusterig, rüsterig, German rostig, Swedish rostig.
more often used for this, or from the general epithet rusty given to various particular firearm names—earlier both was applied in Cockney rhyming slang for other machines, including swords in their day, but the present coinage has not more than a loose connection to this and is from the preference for used or antique firearms due to their being easier or cheaper to obtain.