co-grandfather-in-law

English

co- +‎ grandfather-in-law.

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-à-vis the grandfather of the groom.
- 1661/62, “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”, in Records of Indentured Servants & of Certifications for Land in Northumberland Co. Virginia, page 55:
  "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.

the grandfather of one's grandson- or granddaughter-in-law