Antonyms for "stuck up for" can include "turn against," "abandon," "betray," "criticize," "condemn," "oppose," "disapprove," "reject," and "disavow." These words represent actions that are opposite to defending or supporting someone or something, which is the essence of sticking up for. Turning against or abandoning someone can mean a complete lack of support, while criticizing or condemning suggests a negative opinion or judgment. Opposing and rejecting indicate an active stance against someone or something. Disapproving and disavowing might indicate a more passive rejection, indicating a lack of support or endorsement.