Protected against
rust, either by being made of material that cannot rust, or by having
chemicals or a chemical process applied that prevents the
oxidation of
iron.
In NetHack, rustproofing is good to have, because a rust monster and/or incidents with water will not reduce the effectiveness of the rustproof equipment.
To rustproof a weapon (or remove rust from a rusty weapon), read a scroll of enchant armor while confused.