Edward, Prince of Wales, a British royalty, wearing a happi
A happi (法被, 半被) is a traditional Japanese straight-sleeved coat usually made of indigo or brown cotton and imprinted with a distinctive mon (crest). They are usually worn only to festivals. Originally these represented the crest of a family, as happi were worn by house servants. Later, the coats commonly began to display the crests of shops and organizations. Firefighters in the past also used to wear happi; the symbol on their backs referred to the group with which they were associated. In English, "happi" is most often translated as "happi coat" or "happy coat", although it is pronounced more like "hoppy" in Japanese.
^Drazen, Patrick. Anime explosion!: the what? why? & wow! of Japanese animation. Stone Bridge Press, 2003. ISBN 1-880656-72-8. Page 322. "In time, these groups of fire-fighters, adopting uniforms consisting of the short jackets called happi emblazoned with the mon (crest) of the particular group, so that one gang could be distinguished from another."