“On top of one’s game” literally means “to be at the top of one’s game.”
In other words, it is used to describe someone who is performing at their best in their field or situation.
In Korean, it can be naturally translated as “to be in peak condition,” “to be showing top-level performance,” or “to be in great form.”
This expression is used a lot in everyday situations, and it also appears often in academic and professional contexts.
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✦ Everyday English
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She was really on top of her game during the presentation. → She was really on top of her game during the presentation.
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He’s on top of his game this season. → He’s on top of his game this season.
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I’m totally on top of my game today. → I’m totally on top of my game today.
✦ Academic English
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This study demonstrates that the author is on top of his game in the field. → This study demonstrates that the author is on top of his game in the field.
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The presenter was clearly on top of her game, making full use of the latest data. → The presenter was clearly on top of her game, making full use of the latest data.
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He was on top of his game during the academic discussion, with sharp questions and answers. → He was on top of his game during the academic discussion, with sharp questions and answers.