Utsubushi is a bump that forms on a certain kind of tree.

Because it contains a lot of tannin, it was used for dyeing and medicinal purposes, and was also used for "tooth blackening".

When aphids parasitize the leaves and stems of a tree called Nurde (white glue tree), they form bulges.

The gall is called Fushi, and because it is hollow inside, it is called Utsubushi.