Here's a breakdown:
* Rust: A reddish-brown oxide that forms on iron or steel when exposed to air and moisture. It's a symbol of decay and deterioration.
* Go to rust: This implies that something, whether it be a physical object or a skill or knowledge, is left unused and deteriorates over time.
Examples:
* "If you don't use your language skills, they'll go to rust."
* "That old car has been sitting in the garage for years. It's probably gone to rust by now."
Essentially, "go to rust" means to become useless and forgotten because of inactivity.