Noun
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.