The phrase "knock dead" is often used to describe someone who impresses or amazes others. However, there are several antonyms that could be used to describe a person who does not meet the expectations of others. Some of these antonyms include "disappoint," "underwhelm," "underperform," and "flop." These words imply that the person fell short of expectations and did not leave a positive impression. Other antonyms might include "bore," "disinterest," or "fail to impress." It is important to keep in mind that while these words may be opposites of "knock dead," they are subjective and dependent on individual opinions and perceptions.