hot spring good for the skin
Refers to a hot spring or bath whose water is believed to improve skin condition, often marketed as making one more beautiful.
This inn has an open-air bath called a 'beauty hot spring'.
Compound of 美人 (beautiful woman) + の (possessive particle) + 湯 (hot water/hot spring). The exact origin is uncertain, but it is a common marketing term for hot springs said to benefit the skin.