english
1
general::
Refuse to support, oppose in a dispute, as in The older club members sided against the new program director and her strenuous exercise programs. [First half of 1700s] For the antonym, see SIDE WITH.
American Heritage Idioms
2
general::
Phrase(s): side against someone to be against someone; to take sides against someone. • I thought you were my friend! I never thought you would side against me! • The two brothers were always siding against their sister.
McGrawhill's American Idioms And Phrasal Verbs