The phrase "knocking someone's socks off" means to impress or amaze someone greatly. It is a positive expression that is often used to describe a powerful impact, fantastic performance, or an extraordinary achievement. On the other hand, some antonyms for this expression would be things that are unimpressive, boring, or disappointing. These could include phrases such as "failing to impress," "lackluster performance," "underwhelming accomplishment," "bland production," or "average experience." Such expressions indicate a lack of excitement or the failure to meet expectations. In any case, it is essential to identify the appropriate antonym depending on the context and the extent of the expected impact.