sitz bath; hip bath
A shallow bath where one sits with water covering only the hips and lower abdomen, often used for therapeutic or hygienic purposes.
After giving birth, I was told to keep the wound clean with a sitz bath.
A hip bath is sometimes recommended for treating hemorrhoids.
Compound of 腰 (こし, 'hips, lower back') and 湯 (ゆ, 'hot water'). The exact historical derivation is uncertain, but the term straightforwardly describes a bath that covers the hip area.