Sew up is a phrasal verb that means to close or complete something successfully. The opposite of sew up is to undo or unravel what has been sewn up. Other antonyms for sew up could be to mess up, ruin or botch a task. If something is not sewn up, it could be considered incomplete, unfinished or uncertain. Another antonym would be to leave something open-ended or to leave something to chance. In a literal sense, to tear, rip, unstitch or unfasten could also be antonyms for sew up. Therefore, antonyms for sew up depend on the context in which the phrase is used.