- the father of one's son- or daughter-in-law; that is, the father-in-law of one's son or daughter, or, the father of one spouse in relation to the parents of the other spouse.
- (in the plural) The relationship between people whose children marry each other; the father of the bride vis-à-vis the father of the groom.
- (uncommon) Found primarily in translation. In conversation, the generic "in-law" is generally used, with context left to disambiguate. Once grandchildren are born, the term co-grandfather may be used if the focus is on the relationship through the grandchildren rather than though the married couple.
- co- + father-in-law