장인이 · 장인을
father-in-law (wife's father)
Example
장인어른께서 낚시를 정말 좋아하세요.
My father-in-law really loves fishing.
Word parts
장 elder+인 person
Literally 'elder person'. Homonym warning: another 장인 means 'craftsman'.
Good to know
When talking to or about him politely, say 장인어른.