sleeping outside on a hot summer night
Rare term for the practice of sleeping outdoors to escape indoor heat during summer, often on a veranda or in a garden.
In the past, many households would sleep outside on hot summer nights.
Compound of 外 (そと, 'outside') and 寝 (ね, 'sleep'). The exact historical origin is uncertain, but it straightforwardly describes the act of sleeping outdoors.