noun
taking a bath at someone else's home
Refers to the act of bathing at another person's house, often when one's own bath is unavailable or as a social custom.
昔は近所の家でもらい湯をすることがよくあった。
In the old days, people often took a bath at a neighbor's house.
銭湯ができる前は、もらい湯が一般的だった。
Before public baths existed, bathing at someone else's home was common.
Compound of もらい (receiving) and 湯 (hot water/bath). The exact historical derivation is uncertain, but it straightforwardly describes receiving the favor of a bath.