This is a knitted men's haori with a prominent crest displayed on the back.
Tomoe, commonly translated as "comma", is a comma-like swirl symbol used in Japanese mon. The design became the symbol for water because of its resemblance to water and was worn by firefighters to protect against fires. It has been placed on roof tiles, gables of homes, and temples to ward off fire. Tomoe is the crest of Hachiman, the god of war, and was used as a divine crest in shrines throughout Japan.