木已成舟　

◎ 木已成舟 mùyǐchéngzhōu

[what is done cannot be undone;the wood is already made into a boat] 树木已经制造成舟楫。比喻事已定型,不能改变

如今木已成舟,也是林兄命定如此。——《镜花缘》