noun
a person or thing that is formidable or impressive
preposition
used before 'force' to indicate that it is a force that should be taken seriously or considered
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The new CEO is a force to reckon with in the business world.
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She's definitely a force to reckon with on the basketball court.
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That boxer is a force to reckon with in the ring.
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Her determination and skills make her a force to reckon with in any situation.