sitz bath; hip bath; lower-body bath
A bath where only the lower half of the body is immersed, often for therapeutic or hygienic purposes.
The doctor recommended a sitz bath.
Soaking in a lower-body bath at the hot spring eases my back.
lower bath; lower pool (onsen)
Refers to a bath or pool located at a lower position within a hot spring facility, often contrasted with an upper bath (上湯).
This inn has an upper bath and a lower bath.
The lower bath is a bit on the lukewarm side.
Compound of 下 (しも, 'lower') and 湯 (ゆ, 'hot water'). The exact historical derivation is uncertain, but the word is transparently formed from its components.