noun
grandfather-in-law
Refers to the grandfather of one's spouse, or a step-grandfather by marriage. A formal kinship term.
義祖父は今年で九十歳になります。
My grandfather-in-law will be ninety years old this year.
Biological grandfather; 義祖父 specifies the relationship by marriage.
Compound of 義 (ぎ, 'in-law' or 'non-blood relation') and 祖父 (そふ, 'grandfather').