Plural of 'grand,' commonly referring to thousand-dollar amounts of money, or to grand pianos when used in musical contexts.
From Old French 'grand' (large, great), from Latin 'grandis' (great). In slang, 'grand' for money emerged in the 20th century, likely from 'grand' (impressive/thousand-dollar amount).
The fact that people use 'grands' for money (as in 'he made five grands') shows how slang repurposes adjectives—'grand' means impressive and large, so a 'grand' became street slang for the biggest unit of money people regularly dealt with.
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