1
general::
Support or favor, as in The Armenians traditionally side with the Greeks against the Turks. This idiom was first recorded in 1600. For the antonym, see SIDE AGAINST.
American Heritage Idioms
2
general::
Phrase(s): side with someone
to join with someone; to take someone else’s part; to be on someone’s side. • Why is it that you always side with him when he and I argue? • I never side with anybody. I form my own opinions.
McGrawhill's American Idioms And Phrasal Verbs