Dated term for a toilet, especially a traditional non-flush type. In modern Japanese, トイレ or お手洗い are used instead.
In old houses, the toilet was outside.
Historical kanji spelling; rarely used in modern Japanese. The word itself is dated.
Kana spelling is more common for this dated term, as the kanji is obscure.
The exact derivation is uncertain; the kanji 厠 is a historical Chinese character for toilet, and the reading かわや may be of native origin.