Jacks up is a phrasal verb that means to increase, raise or elevate something. Its antonyms, or opposite meanings, would be to decrease or lower something. Synonyms for this phrase could be to boost, ramp up, or hike up. However, if you are looking to express the opposite meaning, some appropriate options would be to bring down, reduce, or cut. For instance, instead of saying "the company jacked up the prices of its products", one could say "the company reduced the prices of its products to attract more customers". It is important to understand the meanings of antonyms as it can enhance one's communication skills and prevent misunderstandings.