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A proverb meaning that inactivity leads to decline or loss of abilities; one must stay active to keep skills sharp.
Er hat den ganzen Tag nur ferngesehen – ich glaube, er glaubt an das Sprichwort: Wer nicht arbeitet, der rostet.
He spent the whole day just watching TV – I think he believes in the proverb: If you don't work, you rust.
Often used to motivate someone to keep practicing or working.