The word "unclog" means to clear or open up something that was previously blocked or obstructed. Therefore, its antonyms would be words that mean to block or obstruct something. Some possible antonyms for "unclog" might include words like "clog," "block," "jam," or "obstruct." These words would describe actions that might prevent the free flow of water, air, or other substances through pipes or channels. While the word "unclog" is typically used in reference to plumbing or drainage systems, its antonyms can be applicable in a variety of different contexts, such as traffic flow, communication channels, or even emotional blockages.