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early 15c., from grand (adj.) + father (n.), probably on analogy of French grand-père. Replaced grandsire and Old English ealdefæder. Grandfather clause originally (1900) referred to exemptions from post-Reconstruction voting restrictions in the U.S. South for men whose forebears had voted before the Civil War. Grandfather clock is c.1880, from the popular song; they were previously known as tall case clocks or eight-day clocks.
To give someone a special status or privilege because of service before the time a new or definitive arrangement is made: Some farmers just got grandfathered in, that's true
[1900+; fr the grandfather clause often written into new arrangements in order to be fair to older incumbents or practitioners; the date indicates the earliest instance of grandfather clause]