Meaning

adjective covered with or consisting of rust

adjective of the brown color of rust

adjective impaired in skill by neglect

originEllipsis of rusty one more often used for this, or from the general epithet rusty given to various particular firearm names—earlier both were 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.

Etymology from Wiktionary, CC BY-SA 3.0