Covered with rust or affected by rust; also means out of practice or skill at something you once knew.
From 'rust,' which comes from Old English 'rust' and Old Norse 'rost,' relating to the oxidation of iron. The metaphorical meaning (out of practice) developed because skills, like metal, 'corrode' when not used.
It's brilliant that the same word describes both chemical oxidation on metal AND mental degradation from lack of use—your brain really does 'rust' without practice, as neuroscience shows that unused neural pathways actually weaken over time.
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