The antonyms for the phrase "didst bang-up job" could include inadequate, poor, disappointing, unsatisfactory, unacceptable, and substandard. When someone does not do a bang-up job, it means they did not perform exceptionally well. In contrast, using the antonyms in place of the phrase implies that the job was not completed satisfactorily or up to par. These words can be used to provide constructive criticism and highlight areas where improvement is needed. In any task or project, doing a "bang-up job" is the ultimate goal, but sometimes, it can be difficult to achieve.