"Jim-dandy" is an idiomatic expression that refers to something that is excellent or outstanding. However, there are several antonyms for this word that convey the opposite meaning. Some of the antonyms that can be used to describe something that is average or mediocre include average, ordinary, mundane, run-of-the-mill, and humdrum. On the other hand, if we want to describe something that is terrible or subpar, we can use antonyms like awful, lousy, garbage, pathetic or shoddy. These antonyms can be particularly useful when we want to provide a more nuanced or specific description of something that is not necessarily impressive or exceptional.