1
general::
1. Agree with, be in harmony with, as in We happily fell in with his plans. [First half of 1600s] 2. Associate with, become acquainted with (especially by chance), as in On the cruise we fell in with a couple from Boston. [Late 1500s]
American Heritage Idioms
2
general::
Phrase(s): fall in with someone or something
to become involved with someone or a group. • I’m afraid that he fell in with the wrong kind of friends. • John fell in with Max, who had served time in prison., Phrase(s): fall in with something
to concur with something; to harmonize with something. • We had to fall in with her wishes. • The statement falls in exactly with my view.
McGrawhill's American Idioms And Phrasal Verbs