iris-leaf bath; sweet-flag bath
A traditional bath infused with iris (sweet flag) leaves, especially taken on May 5th (Children's Day) for health and protection.
There is a custom of taking an iris-leaf bath on Children's Day.
Compound of 菖蒲 (shōbu, 'sweet flag/iris') and 湯 (yu, 'hot water/bath'). The practice is associated with the Boy's Festival (now Children's Day) and is believed to ward off evil and promote health.