Moukohan

Moukohan (Mongolian spots) is a blue spot on the lower back of Asian infants and babies. The blue colour is caused by Melanin-containing cells – Melanocytes that are deep under the skin. The spots usually disappear in a few years and almost always by puberty. In Japan these spots led to an idiom "his butt is blue" ("shiri ga aoi"), which means a person is childish.

References

• Japan SAQ – seldom asked questions about Japan