noun
man who marries into his wife's family; adopted son-in-law
Refers to a man who takes his wife's family name and becomes a member of her household, often in a traditional family succession context.
彼は入り婿として妻の家に入った。
He entered his wife's family as an adopted son-in-law.
入り婿の制度は、家を継ぐために昔からある。
The custom of the adopted son-in-law has existed since old times to carry on the family line.
Compound of 入り (iri, 'entering') + 婿 (muko, 'son-in-law'). The exact historical derivation is uncertain, but the word has been used to describe a man who marries into his wife's family.