"Knock for six" is a popular phrase that means to completely shock or overwhelm someone. There are several synonyms that could be used to replace this phrase, depending on the context. For example, the term "blow away" could be used to describe a situation where someone is completely taken aback or stunned. Another synonym could be "knock off their feet," which implies that someone is completely knocked down or overcome by surprise. Other options might include "astound" or "floor," both of which can be used to describe situations where someone is completely overwhelmed by an event or situation. Ultimately, choosing the right synonym will depend on the context and intended meaning of the phrase.