There is actually nothing wrong in addressing the grandfather with the word abi, abb, abu, or the like. If you refer to the Quran in Arabic, you will find in Surat al-Baqarah 2:133 the Arabic word abaa` (plural of abb (father)) is used for the father (Isaac), grandfather (Abraham) and even referring to the uncle (Ishmael).
Is Addressing a Person’s Father with the Arabic Word ‘Aba’ Permissible?
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