A shallow bath in a bathhouse or hot spring where you can lie down and relax. Similar to 寝湯.
See also: 寝湯
This hot spring has a lie-down bath where you can soak while lying down.
Compound of 寝 (ne, 'sleep' or 'lie down') and 風呂 (furo, 'bath'). The exact origin is uncertain, but it likely arose in the context of modern bathhouse and hot spring facilities.