Clean is an adjective that describes something free from dirt, pollution, or any type of impurity. Its antonyms are words that describe the opposite of clean. One such antonym is 'dirty,' which means something covered in dirt, grime, or dust. Another antonym is 'filthy,' which describes something extremely dirty and unpleasant. 'Unclean' is a synonym of dirty and it means not clean, impure or contaminated. 'Contaminated,' itself an antonym of clean, means something that has been exposed to infectious agents, harmful chemicals or pollutants. 'Foul' is yet another antonym, describing something unpleasant or disgusting, and 'grimy' denotes something covered in dirt and blackened grime. Lastly, 'messy' means disordered, cluttered, or untidy.