An informal or dialectal shortening of grandfather.
From grandfather with the final syllable -ther reduced to -fer through informal speech patterns. Many English words have informal shortened versions (like 'vocab' for vocabulary or 'prom' for promenade), especially in families and colloquial speech.
Different English-speaking regions have their own informal versions of 'grandfather'—some say 'grandpa,' 'granddad,' 'grandpops,' or 'grandfer.' These nicknames show how family relationships create their own private language, and children often invent pronunciations based on what they hear and can say.
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