"Fill the bill" is an idiomatic expression that typically means "to meet the necessary requirements or qualifications for a particular task or purpose." Antonyms for this phrase could include "fall short", "miss the mark", "be inadequate", "disappoint", or "fail to satisfy." Essentially, any word or phrase that conveys the opposite meaning of being a suitable fit for the requirements or expectations at hand could be considered an antonym for "fill the bill." It is important to note, however, that the exact opposite may depend on the specific context and intended meaning of the phrase in question.