The phrase "keeps in check" refers to controlling or limiting something. Its antonyms would be words that suggest freeing, releasing, or allowing it to expand. One antonym is "lets loose," which implies that there are no restrictions or limitations. Another is "unleashes," which has a similar meaning to letting something out of a cage. "Unbridled" is another opposite of "keeps in check," meaning uncontrolled, unchecked, or unrestrained. Finally, "uninhibited" is an antonym that suggests a lack of inhibition, restraint, or control. By knowing these antonyms, one can better understand the different shades of meaning implied by the phrase "keeps in check.