Co-grandfather-in-law

Full definition of co-grandfather-in-law

(plural co-grandfathers-in-law)

(rare, nonstandard) the grandfather of one's grandson- or granddaughter-in-law; that is, the grandfather of one spouse in relation to the grandparents of the other spouse.(in the plural) The relationship between men whose grandchildren marry each other; the grandfather of the bride vis-Ã -vis the grandfather of the groom.

1661/62, , Records of Indentured Servants & of Certifications for Land in Northumberland Co. Virginia Chapter These are to certify that Rich. Nelmes made his rt. appeare to 200 acs. of land for the transportation of fower persons into this Collony: John Duggin, John Bound, Joyce Mattux & Mary Floyd, "PEMBRIDGE" 300 ACRES ON THE SOUTHERN SIDE OF NANTICOKE RIVER SOUTH SIDE OF A CREEK ISSUING FROM SAID RIVER CALLED "DEEP CREEK." LYING BETWEEN LANDS OF LAWRENCE YOUNG (SON-IN-LAW) AND WILLIAM COLLIER (ASSUMED TO BE CO-GRANDFATHER (IN-LAW) OF ELIZA DAUGHTER OF ELDEST SON WILLIAM BOUNDS)
ACQUIRED BEFORE 1688 WHEN IT WAS IN THE POSSESSION OF SON WILLIAM BOUNDS. THIS 1688 DATE ESTABLISHES A BIRTHDATE OF AT LEAST 1667 FOR WILLIAM, AS HAD HE BEEN UNDER 21 YEARS OF AGE HE COULD NOT HAVE POSSESSION OF "PEMBRIDGE".

Rare in conversation, the generic in-law or grandfather is often used, with context left to disambiguate.