Grandfather
wiktionary
The noun is derived from Middle English grandfadre, graundfadir, graunfadir, grauntfader, and other forms, from graunt(“big, large; great, important”) [1] + fā̆der(“male parent, father; remoter male ancestor”), [2] probably modelled after Middle French grandpere, grant pere(“male parent; remoter male ancestor”) (whence French grand-père); [3] the English word is analysable as grand- + father. Superseded earlier eldfather, elderfather. [3]
The verb is derived from the noun. [4]
From grandfather clause: see grandfather ( etymology 1) and clause.
etymonline
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.
