Rare term for a changing room or locker room, typically in a public bath, gym, or pool. More commonly expressed as 更衣室 (こういしつ).
I changed clothes in the hot spring's changing room.
Compound of 更衣 (こうい, 'changing clothes') and 場 (じょう, 'place'). The exact historical derivation is uncertain, but the spelling is conventionally associated with a place for changing clothes.