also: かっぺき
noun
next-door neighbor; neighbor sharing a wall
Refers to a neighbor whose house shares a common wall, as in a row house or apartment. Somewhat literary or descriptive; in everyday speech 隣人 or 隣の家 are more common.
合壁の家から話し声が聞こえてくる。
I can hear voices from the house next door.
合壁同士で壁の補修について話し合った。
The neighbors sharing a wall discussed repairing it.
Compound of 合 (together) and 壁 (wall), literally 'shared wall'. The reading がっぺき is a standard rendaku form; かっぺき is an older unvoiced variant.