The father of one's mother or father.
A forefather; an ancestor.
To exempt (one involved in an activity or business) from new regulations: The new ordinance restricts the size of billboards, but it grandfathers those erected before 1997.
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.