The term magnum opus refers to a person's greatest work or a masterpiece. Antonyms, on the other hand, are words that have the opposite meaning of a particular word. Therefore, antonyms for magnum opus include mediocre work, amateurish endeavor, poor performance, subpar creation, and lackluster project. These words indicate that the work being referred to is not of good quality or is below average in terms of creativity, skill, or execution. Such pieces may lack the caliber, depth, or impact that is expected of a magnum opus. Therefore, using antonyms for magnum opus can help to convey a lower level of attainment and a less extraordinary achievement.