He wasn't at his best today and only showed his C-game.

C-game 是由字母 C 和 game（比赛、表现）组成的复合词。在英语中常用字母等级（A/B/C/D）来评价表现水平，A-game 指最佳状态，C-game 则表示"低于平常水平的发挥"或"三流表现"。