fall in with
fall in with {v.}, {informal}
1. To meet by accident.
Mary fell in with some of her friends downtown.
2. To agree to help with; support.
I fell in with Jack's plan to play a trick on his father.
3. To become associated with a group detrimental to the newcomer.
John fell in with a wild bunch; small wonder he flunked all of his courses.
Compare: PLAY ALONG.
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Source: A Dictionary of American Idioms