Sentence:  It also overlooks the fact that in a rational lexicon , and quite clearly in More 's lexicon , the opposite of serious is not gay but frivolous , and the <WSD>opposite</WSD> of gay is not serious but solemn . 
Meaning of the word marked with <WSD> tags:  a word that expresses a meaning opposed to the meaning of another word, in which case the two words are antonyms of each other