A game

American  

Or A-game,

noun

Informal.

1. a person’s best effort, abilities, or achievement.

   The candidate brought her A game to the debate.

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'A' game British  

noun

1. informal one's best possible performance, esp in the phrase to bring or take one's 'A' game

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Etymology

Origin of A game

In reference to a sports contest, with the letter A indicating peak performance

Example Sentences

He made his return to rugby in an England A game against France, but earned a red card for a high shot on Antoine Hastoy in the 34th minute.

From BBC

The British number one, whose sole title remains her astonishing 2021 US Open triumph, rushed through the first set in 25 minutes without losing a game.

From BBC

“It’s kind of a game,” I explained.

From Literature

Gilgeous-Alexander is averaging 31.8 points, 6.4 assists and 4.4 rebounds a game for Oklahoma City, which has the NBA's best record at 40-11.

From Barron's

Doncic leads the NBA in scoring this season with 33.4 points a game.

From Los Angeles Times