Definition from Wiktionary, the free dictionary
co-grandfather (plural co-grandfathers)
- 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”, Time:
- 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)