off the rails

详细解释:出轨的, 混乱的, 神经错乱的

例句：

I would never have guessed that Mary of all people would go off the rails, and her she is seeing another man behind Jim's back.我过去绝不会猜到，偏偏是玛丽会行为越轨，竟然背着吉姆与另一个男人幽会。