The phrase 'most whacking' is often used to describe something that is extremely large, impressive or powerful. Synonyms for this phrase include 'enormous', 'colossal', 'massive', 'immense', 'gigantic', and 'monumental'. Other synonyms include 'superlative', 'top-notch', 'first-rate', 'outstanding' and 'exceptional'. These words can be used to describe anything from a person's physical size to the impact of a powerful speech or the magnitude of a natural disaster. In summary, 'most whacking' is a colorful and informal way of expressing the idea of something being large or impactful, and there are a wide range of synonyms that can be used to convey this meaning.