In general, the term ‘grandfather’ is not considered a proper noun. Rather, it functions as a common noun because it refers to any grandfather, not a specific one. Proper nouns are specific names of people, places, or organizations, such as ‘Grandpa Joe’ or ‘Grandfather Smith,’ which denote particular individuals.
When we refer to someone as ‘my grandfather,’ we are using the term generically to describe the familial relationship rather than pointing to a specific person named ‘Grandfather.’ However, if ‘Grandfather’ is used as part of a title or a specific name, it could become a proper noun in that context.
For example, in a story, if a character is named ‘Grandfather’ as part of a title, then it is treated as a proper noun. Context plays a crucial role in determining whether a noun is proper or common.