Decategorization Theory provides a theoretical framework for elaborating the non-prescriptive usage of English nouns, including its conversion of countability, adjectivalization, and adverbalization. There would be some semantic and syntactic changes of categorical attributes after decategorization of English nouns, acquring some new grammatical functions. As the working mechanism of Decategorization Theory, metonymic cognition contributes to the cognitive processing and semantic construction of non-prescriptive usage of English nouns. As a way of creative thinking, decategorization is the internal cognitive motivation of non-prescriptive usage of English nouns, while pursuing economy in expression is its external pragmatic motivation.