A Toss-Up
指结果不确定，双方机会均等，难以预测胜负或选择。
描述竞争或选择结果难定时使用
英文释义
an uncertain or unpredictable outcome; a situation in which the result is unclear or equally likely
例句
The election is a toss-up; neither candidate has a clear lead.
典故来源
源自抛硬币决定胜负的习俗，硬币落下前结果未知，引申为不确定。