In the movie "Dreamer", Dacota said, "Pop said this used to be very beautiful." This 'pop' meant "granpa". In the dictionary I couldn't find that "pop" means "granpa". Do you use "pop" as a grandpa? Thank you.
I don't know whether it's common, but 'pop' may be used as 'grandpa' at your discretion. For example, I have a friend who is used to calling her grandmother 'mum'.
Pop usually means father, but it could easily be used as grandfather. Also: poppa and pa.

A lot of families have unique words for their grandparents, sometimes coming from the language of the families' heritage. The show "Arrested Development" parodies this by having rather strange names: Pop-pop and Genki.
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I understand pop simetimes means grandpa. Thank you all!
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