adult son living at home without inheritance; dependent adult child
Historically referred to an eldest son who had not yet inherited the family estate and remained dependent on his parents. In modern usage, it can describe any adult child still living at home and financially dependent, often with a slightly negative or pitiful connotation.
In the Edo period, the eldest son who was a heyazumi could not inherit the family headship until his father retired.
He is over thirty and still a heyazumi, living off his parents.
Compound of 部屋 (heya, 'room') and 住み (sumi, 'living'), literally 'room-dwelling'. The term originated in the Edo period to describe an adult son who remained in his parents' home without his own household.