co-grandfather

• co- +‎ grandfather

co-grandfather (plural co-grandfathers)

1. the father of one's son- or daughter-in-law, as common grandfather of their children; the relationship between men who have grandchildren in common; maternal grandfather vis-à-vis paternal grandfather.
   • 1946 September 9, “The Milkman”, in Time‎, archived from the original on 19 February 2011:

     The milkman was Edward Kenelm Digby, 52, eleventh Baron Digby, World War I colonel in the Coldstream Guards, World War II inspector of infantry-training establishments, co-grandfather (with Winston Churchill) of Randolph Churchill's small son, Winston Churchill II.

• co-father-in-law (relative to the children rather than the grandchildren)

• co-grandmother
• co-grandparents

• co-grandfather-in-law
• co-in-law
• co-uncle