staying at the same inn; rooming together
Refers to the act of sharing accommodation, typically at an inn or hotel. Often used in historical or travel contexts.
I ended up staying at the same inn as him during the trip.
相宿の客と夜遅くまで話し込んだ。
I talked late into the night with a fellow guest at the inn.
Compound of 相 (あい, mutual) and 宿 (やど, inn/lodging). The exact historical derivation is uncertain, but the word is conventionally associated with the idea of sharing the same temporary lodging.