「夜 に 唇 を 吹く と 蛇 が 来る」 - Yoru ni kuchibiru wo fuku to hebi ga kuru
"If you whistle at night, a snake will come at you"
During the Edo period (also known as the Tokugawa period, the historical period between 1603 and 1868), thieves used to communicate and warn each other by whistling. As a result, whistling at night nowadays is associated with inviting burglars (the snake) into your place.
