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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.
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.
To align oneself with someone or something in a disagreement: The stalemate ended when a small group of senators sided with the opposing party.