"Not too shabby" is an informal phrase that means "pretty good" or "acceptable." Some antonyms for this phrase include "terrible," "atrocious," "lousy," "dreadful," and "horrendous." These words emphasize a negative connotation and indicate a significant lack of quality, whereas "not too shabby" suggests there is adequate quality, but not necessarily exceptional. Other antonyms for "not too shabby" could be "mediocre," "average," "ordinary," or "unsatisfactory." These words suggest that the quality is neither bad nor good, but somewhere in the middle of the spectrum. Overall, using different antonyms can change the tone and meaning of the original phrase.