不共戴天是指不愿和仇敌在一个天底下并存。形容仇恨极深。

出自《礼记·曲礼上》：“父之仇，弗与共戴天。”

那么不共戴天用英语怎么说？

不共戴天可以用absolutely irreconcilable来表示，例如：

1.俄罗斯总理弗拉基米尔·普京，一个与中欧导弹防御系统不共戴天的人，赞扬美国放弃欧洲盾牌的决定“正确又勇敢”。
The Russian prime minister, Vladimir Putin, an implacable opponent of the central European missile shield, praised the US decision to drop the European shield as "correct and bold". 

2.一些富有远见的科学家曾经是不共戴天的敌人，他们就像圣经中的诗句所说“化干戈为玉帛”，为此携手破解难题。
Ultimately, that's what is meant by the verse in the Bible about "beating swords into plowshares." It took some visionary scientists--once mortal enemies--to join hands and figure it out. 

3.“必须写完一份关于19世纪封建军阀的报告，”鲁思说，“到底什么是不共戴天之仇？”
“I have to finish my report on the feudal warlords of the 19th century,” Ruth said. “What is a feud, anyway?”

4.在《暮光之城3：月蚀》的剧情里，克里斯汀·斯图尔特扮演的女主角贝拉再度（毫无悬念滴）陷入了危难之中。 西雅图发生的一系列离奇命案，都可能是她有着不共戴天之仇的恶毒吸血鬼的报复行为。
In Eclipse, Bella (Kristen Stewart) once again finds herself surrounded by danger as Seattle is ravaged by a string of mysterious killings and a malicious vampire continues her quest for revenge. 

5.她是我不共戴天的仇敌！
She is my absolutely irreconcilable enemy!