The phrase "sets loose" refers to freeing or releasing something from its confines. Antonyms for this phrase would imply keeping something restrained or contained. Some possible antonyms include confine, restrict, restrain, imprison, and control. These words suggest that the subject is kept in check, and not allowed to roam free. For instance, if someone "sets loose" a dog, their antonym would keep the dog on a leash or in a kennel. Similarly, if a prisoner is "set loose," the antonym would keep them behind bars or under watch. Ultimately, the antonyms for "sets loose" denote a sense of confinement, whereas the original phrase implies release and freedom.