noun
A shallow bath in a bathhouse or hot spring designed for reclining, often with a headrest.
温泉の寝湯でゆっくり休んだ。
I relaxed in the lie-down bath at the hot spring.
この施設には寝湯と露天風呂がある。
This facility has a lie-down bath and an open-air bath.
Compound of 寝 (ne, 'sleep' or 'lie down') and 湯 (yu, 'hot water'). The exact historical derivation is uncertain, but the term straightforwardly describes a bath for reclining.