An idiomatic exhortation meaning 'beware of fire' or 'be careful with fire' — used to remind people to prevent fires, especially in dry seasons or festivals.
冬は乾燥しているから、夜は必ず火の用心をするようにと言われた。
Because winter is dry, I was told to always be careful about fire at night.
町内会が回ってきて、提灯を掲げながら『火の用心』と叫んで巡回した。
The neighborhood association went around, patrolling while carrying lanterns and shouting 'Be careful with fire.'
ストーブを消し忘れないか心配で、寝る前に妻に『火の用心してね』と声をかけた。
Worried about leaving the stove on, I told my wife before bed, 'Be careful with the fire.'