An archaic or poetic term for a grandfather or an aged male ancestor; sometimes used respectfully in older literature.
From grand- plus sir (from Old French sire, meaning lord or authority figure). This compound is largely obsolete, surviving mainly in historical texts and period literature from the Medieval and Early Modern periods.
Grandsir is the kind of word you find in Shakespeare and old novels—it shows how English used to respect age by literally calling it 'grand-sir,' while modern English just says 'grandpa'!
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