"Cats pajamas" is an idiomatic phrase that means something or someone is really great, outstanding, or the best. It is unclear why this phrase became popular in the 1920s, but it quickly became a part of the common vocabulary. To find antonyms for the phrase "cats pajamas," one must look for words that mean unremarkable, average, or disappointing. Some antonyms for "cats pajamas" could include "mediocre," "ordinary," "average," "unexciting," or "unspectacular." These words would describe a person, thing, or situation that is not outstanding or impressive.