The phrase "滑天下之大稽" is formed by using the verb 滑, which here functions as "to engage in" or "to become", followed by the noun phrase 天下之大稽, where 天下 means "the world", 之 is a possessive particle linking the elements, and 大稽 means "great ridicule", so the entire structure literally combines to express "to become the world's greatest ridicule" or to represent something as the ultimate absurdity.