cross the line字面意思越过某条界线，一般是指触犯了某项禁忌或者规则，做了过分的事情，即越界，做得过火，超过权限。

【例句】
Because my father would never cross the line like that.
因为我爸爸从来不会干那么出格的事。

Solving crimes won't be enough. One day he'll cross the line.
他不会满足于破案，总有一天他会越过底线。