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.
to agree with or support someone take somebody's sideWhen workers have sued companies for violating their privacy, judges have usually sided with the employer.
Usage notes: the opposite meaning is expressed by side against someone(to disagree with or not support someone): As much as I hate to side against a friend, I feel she is wrong.
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.