win是“赢、胜利”的意思，那win by a hair字面意思就是“通过一根头发取胜”，说明胜利是险胜，以毫厘之差取得的胜利。

【例句】
Nancy ran faster than ever before but she lost by a hair.
Nancy比以往任何时候都跑得快，但是她还是以毫厘之差败北。

It was as close as an election could be. A million people voted and the candidate I voted for won by only 350 votes. She only won by a hair but that still makes her our new governor.
这次选举竞争空前激烈。有一百多万人投票，我选的侯选人仅仅领先三百五十票而赢得胜利，她以微弱的优势险胜当上了州长。