The word 堵塞 is a compound formed by combining two characters with similar meanings to create a stronger, more specific term. The first character, 堵 (dǔ), primarily means "to stop up" or "to block", often referring to an action that causes obstruction. The second character, 塞 (sè/sāi), means "to plug", "to fill in", or "to cram", emphasizing the state of being clogged or impassable. When combined, they create a verb-noun pair that synergistically denotes the act or result of blocking up a passage, such as a pipe, road, or artery, where 堵 implies the causing action and 塞 emphasizes the resulting congested state. This formation follows a common pattern in Chinese where synonymous or related morphemes are paired to reinforce and specify a concept.