The antonyms for the phrase "giving thumbs up" are varied and can include several gestures and expressions that convey the opposite sentiment. For instance, "giving thumbs down" is a symbol of disapproval, negativity, and rejection. Other antonyms could include ignoring, denying, disliking, not supporting, or even sabotaging someone or something. It's essential to remember the context and the situation when using these antonyms, as they might carry different meanings or implications depending on the circumstances. Overall, while giving thumbs up is a sign of approval and agreement, its antonyms express the opposite feelings and attitudes.