go off the rails
US /ɡoʊ ɔf ðə reɪlz/
UK /ɡoʊ ɔf ðə reɪlz/
1.
to start behaving in a way that is not acceptable or expected, especially in a wild or uncontrolled way
:
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After his parents' divorce, he really started to go off the rails.
•
The project seemed to go off the rails when the main manager left.