a person who uses or gives money, especially someone who spends it freely or in large amounts.
From 'spend' (Old English 'spendah' from Latin 'expendere' meaning 'to weigh out') plus agent suffix '-er' to describe the person who spends.
The phrase 'big spender' became a thing in American English in the early 1900s as a way to describe wealthy people who weren't shy about showing off their money—it's got this mix of admiration and judgment baked into it.
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