The phrase "come undone" means to fall apart or become unraveled, and some antonyms for this phrase would be to come together or become whole. Other antonyms might include to fix, repair or mend. To prevent something from "coming undone," one might take steps to secure or attached something firmly, such as by tightening a bolt or knotting a rope. Antonyms that convey stability might include to remain in place, hold together, or stay intact. Overall, antonyms for "come undone" suggest a sense of strength, stability, and reliability.