n the old times, whistling was a sign used by burglars and other criminals to communicate with each other. In Japanese a “night burglar” has their own special word which is yatou (夜盗/やとう).
You may recognize the kanji as the first character (pronounced “ya“) means “night” and the second kanji character (pronounced “tō“) means “steal” or “rob”. Whistling was soon associated with intruders, thieves, and other villains. It was said that whistling at night would attract these villains — or a snake — into your home.