Taking a look at several languages (Basque, Catalan, Italian, Esperanto, Romanish etc) as well as original Latin I would say “grooved”. A ruga is a wrinkle or furrow (implying a worn-in depression) from the Latin, so for me the most apposite meaning for an arruginated key indicates a key worn with use so as to develop grooves, though it could also mean that the metal had developed a bumpy texture due to