The term "jack something up" implies raising or increasing the value or intensity of something. Therefore, some potential antonyms for this phrase could include terms like "lower," "reduce," or "minimize." Other antonyms might include words like "calm," "soothe," or "mitigate," which imply the opposite of increasing or intensifying something. Additionally, antonyms for this phrase may include terms like "prevent," "stop," or "hinder," which imply a desire to inhibit or obstruct something from happening. Regardless of the specific antonym used, it is clear that "jack something up" generally refers to a desire to increase or intensify something, and that its antonyms will similarly relate to efforts to reduce, inhibit, or calm.