Rusty
Old English rūstig (see rust, -y1).
wiktionary
From Middle English rusty, from Old English rūstiġ(“rusty”), from Proto-Germanic *rustagaz(“rusty”), equivalent to rust + -y. Cognate with Saterland Frisian rusterch(“rusty”), West Frisian rustich, roastich(“rusty”), Dutch roestig(“rusty”), German Low German rusterig, rüsterig(“rusty”), German rostig(“rusty”), Swedish rostig(“rusty”).
Ellipsis of rusty one 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.
Variant form of resty; compare also reasty.
etymonline
rusty (adj.)
"covered or affected with rust, rusted," Old English rustig; see rust (n.) + -y (2). Cognate with Frisian roastich, Middle Dutch roestich, Dutch roestig, Old High German rostag, German rostig.
"In the 16th and 17th centuries frequently used as a term of general disparagement" [OED]. In plant an animal names, "having the color of rust." Of bodily skills, "impaired by neglect," from c. 1500; extended to mental qualities, learning, skills, accomplishments, etc., by 1796. Related: Rustily; rustiness.
