[n] the father of your father or mother
gramps, grandad, granddad, granddaddy, grandpa
centenarian, dotard, elder, Father Time, gaffer, geezer, golden-ager, grampa, gramps, grandpa, grandsire, graybeard, great-grandfather, Methuselah, Nestor, nonagenarian, octogenarian, old chap, old codger, old dog, old duffer, old geezer, old gent, old ge