grandpa 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.

a special use of grand (adj.) in genealogical compounds, originally with the sense of "a generation older than," first attested c. 1200, in Anglo-French graund dame "grandmother," also grandsire (late 13c.), from such use of Old French grand-, which perhaps is modeled on Latin avunculus magnus "great uncle." The partly-Englished grandmother, grandfather are from 15c. Other such words in European languages are formed with the adjectives for "old" or "best" (Danish bedstefar) or as diminutives or pet names (Greek pappos, Welsh taid). The French formation also is the model for such words in German and Dutch. Spanish abuelo is from Latin avus "grandfather" (from PIE *awo- "adult male relative other than the father;" see uncle), via Vulgar Latin *aviolus, a diminutive or adjective substitution for the noun.

The extension of the sense to corresponding relationships of descent, "a generation younger than" (grandson, granddaughter) is from Elizabethan times. The inherited PIE root, *nepot- "grandchild" (see nephew) has shifted to "nephew; niece" in English and other languages (Spanish nieto, nieta). Old English used suna sunu ("son's son"), dohtor sunu ("son's daughter").