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'-osity' is a suffix for abstract nouns made from adjectives ending in '-ous.'

Literal

'-osity' [topic-は] ending [subject-が] '-ous' [possessive-の] adjectives [from-から] made abstract-noun [possessive-の] suffix.

語尾 'word ending/suffix,' 形容詞 'adjective,' 抽象名詞 'abstract noun' — all standard linguistic terminology. The sentence explains English morphology in Japanese. A grammar textbook definition.