Definitions

1. adjective

having or becoming rusty; less shiny or less resistant to corrosion

“The old bike had a rustier chain than the one on the new bike.”

2. comparative

comparing two things, the one described is less shiny or less resistant to corrosion

“This engine is rustier than the one in my last car.”

3. adverb

to a greater extent, becoming or having rust

“The metal was getting rustier by the day.”