The phrase "sticks up for" means to defend, support or advocate for someone or something. The antonyms for this phrase would be words that indicate a lack of support, such as ignore, abandon, neglect, betray or criticize. Using these antonyms would mean that instead of advocating for someone or something, you are backing away from them, leaving them unprotected or even actively working against them. Choosing the right antonym for "sticks up for" will depend on the context you are using it in and whether you want to convey a positive or negative message.