Definition from Wiktionary, the free dictionary
- the parent of one's son- or daughter-in-law, as a common grandparent to one's children
2009 May 19, Beth Teitell, “Horns Come Out When Grandparents Envy Each Other”, in Boston Globe, page B7:
- Most grandparents aren't engaged in a duel with their co-grandparents, but the desire to be the most beloved, most generous grandparent often simmers beneath the surface. […] “It can get to the point where they can't speak to the co-grandparents, or the parents can't have events where they're both there,” she said.
- co-parents-in-law (relative to the children rather than to the grandchildren)