word-forming element denoting "kinship one degree further removed," early 15c. (in great uncle), from great (adj.), based on similar use of French grand (see grand-). An Old English way of saying "great-grandfather" was þridda fæder, literally "third father;" in early Middle English furþur ealdefader was used (12c.).
grandfather n.
early 15c., from grand- + father (n.), probably on analogy of French grand-père. Replaced grandsire and Old English ealdefæder. Grandfather clause originally (1899) referred to exemptions from post-Reconstruction voting restrictions (literacy, property tax) in the U.S. South for men whose forebears had had the right to vote before 1867 (thus allowing poor and illiterate whites to continue to vote). Grandfather clock is from 1894, originally grandfather's clock (1876), "a furniture dealer's name" [OED] from "My Grandfather's Clock," the 1876 song by Henry Clay Work that was enormously popular (and loathed) in late 1870s. It indicates that they were beginning to seem old-fashioned; they were previously known as tall case clocks or eight-day clocks.