An in-law is someone who is a relative because of marriage, like your husband's sister or your wife's father. You can refer to your spouse's entire family as your in-laws. ... In-law originally meant "anyone of a relationship not natural" or "not by blood."

What does it mean when someone says in-laws?

: a person you are related to because of your marriage especially : the father or mother of your husband or wife.

Why do we say in-laws?

It comes from Middle English modyr in lawe. As far as I understand it, the term was first used in the 14th or 15th century. The idea behind it is that your mother-in-law has the same rights and duties as your biological mother and is given these rights and duties by the legal pact of marriage.