An informal, affectionate term for a grandfather; a granddad or grandpa.
From grand- plus pap, which itself comes from papa (an infantilized form of father from Latin papas). The double p in grandpap shows it's a casual, spoken form rather than formal writing.
Grandpap is especially common in Appalachian English and some Southern US dialects, showing how different regions preserve different family terms—it's like regional vocabulary fossils!
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