Antonyms for "butter up" include words or phrases like criticize, insult, denounce, humiliate, rebuke, disparage, condemn, or chastise. These terms all represent actions that are quite opposite to "buttering up" someone, which often involves flattery or the use of sweet words and compliments to gain favor or manipulate someone. While "buttering up" can be an effective tactic in certain situations, using its antonyms can be equally effective in others. For example, if you need to criticize someone constructively or offer honest feedback, using antonyms for "butter up" could help to convey your message in a more direct and serious manner.