The word "滂沱" is formed by combining two characters, each containing the water radical (氵) to emphasize association with liquid. "滂" uses the phonetic component "旁" and denotes pouring or torrential water, while "沱" uses the phonetic component "它" and refers to swirling or flowing water. Together, they create a reduplicative compound where the repeated water-related elements intensify the meaning to describe heavy, continuous downpour or copious flowing.