A person or thing that doubles something; a device or substance that increases something twofold, or in card games, a player who doubles a bid.
From 'double' plus the agent suffix '-er.' The word has been used since the 16th century for people who double amounts, and took on card-game meaning by the 18th century.
In bridge, the doubler is like a poker player pushing all-in—by doubling the opponent's bid, you're saying 'I'm so confident you're wrong that I'll bet your points against you,' and if you're right, you earn extra points as a reward.
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