Thinner or more slender in appearance; or a person attempting to lose weight or become thinner through diet and exercise.
From 'slim' (possibly from Dutch 'slim' meaning bad or from German 'schlimm') plus comparative suffix '-er.' Combined with weight-loss meanings in the 20th century.
The modern 'slim' body ideal is actually historically recent—in the 1500s-1800s, being plump was a sign of wealth and beauty because only rich people could afford enough food!
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