The decade from 1930 to 1939, or someone's age between 30 and 39 years old.
Thirties is the plural of thirty, from Old English 'þrittig,' related to three groups of ten. When referring to a decade, it became standard in the 20th century.
The 1930s were called the 'Great Depression' and 'Dirty Thirties'—the decade's nickname changed based on what part of the world you lived in, showing how one time period can have completely different meanings!
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