break in
break in {v.} (stress on "in")
1a. To break from outside.
The firemen broke in the door of the burning house.
1b. To enter by force or unlawfully.
Thieves broke in while the family was away.
2. To enter suddenly or interrupt.
A stranger broke in on the meeting without knocking.
The secretary broke in to say that a telegram had arrived.
Compare: CUT IN (2).
3. To make a start in a line of work or with a company or association; begin a new job.
He broke in as a baseball player with a minor league.
4. To teach the skills of a new job or activity to.
An assistant foreman broke in the new man as a machine operator.
5. To lessen the stiffness or newness of by use.
He broke in a new pair of shoes.
Breaking in a new car requires careful driving at moderate speeds.
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Source: A Dictionary of American Idioms