settle down
settle down {v.}
1. To live more quietly and sensibly; have a regular place to live and a regular job; stop acting wildly or carelessly, especially by growing up.
John will settle down after he gets a job and gets married.
2. To become quiet, calm, or comfortable.
Father settled down with the newspaper.
The house settled down for the night after the children were put to bed.
The teacher told the students to settle down and study the lesson.
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Source: A Dictionary of American Idioms