1.
beware of fire; fire safety; fire prevention
A warning phrase about fire safety. Most commonly heard as a call during traditional neighborhood fire-watch patrols, where volunteers walk through residential areas at night, clapping wooden blocks and calling out this phrase.
Cultural note
A phrase deeply embedded in Japanese culture. During the dry winter months, neighborhood volunteers (夜回り) walk through residential streets at night, clapping 拍子木 (wooden clappers) and calling out 火の用心. The full traditional call is often: 火の用心、マッチ一本火事のもと (Beware of fire — a single match can start a blaze).
Usage
Also seen on signs and posters as a general fire-safety warning.