The antonyms for the phrase "bang up" can be described as peaceful or uneventful. Some of the antonyms for this term include gentle, calm, quiet, serene, tranquil, still, and silent. These words suggest a sense of tranquility and relaxation rather than the sudden, violent impact that is associated with the phrase "bang up." Other antonyms for "bang up" could include mellow, mild, unhurried, slow, or easygoing. These words emphasize a sense of ease and comfort rather than the excitement and intensity of a bang up experience. When it comes to describing the opposite of "bang up," peaceful and calm words seem to be the most fitting antonyms.