Winning an Olympic gold medal was his once-in-a-lifetime achievement.

once-in-a-lifetime 是由三个单词组成的复合形容词，once 表示"一次"，lifetime 表示"一生"，中间用连字符连接。整个短语字面意思是"一生只有一次的"，用来形容极其难得、独一无二的经历或成就。