expression, godan verb (-ru)
err in life; take the wrong path; go astray
A fixed expression meaning to make a serious mistake in one's life choices, leading to ruin or a bad path. Often used in moral or cautionary contexts.
若いときに身を誤ると、後で後悔することになる。
If you go astray when you're young, you'll regret it later.
彼は
He fell in with bad company and ruined his life.