noun
Describes a fire that is burning low, or figuratively, a trend, excitement, or activity that is losing strength. Often used with になる or だ.
火事はようやく下火になった。
The fire has finally died down.
ブームはもう下火だ。
The boom is already waning.
Compound of 下 (した, 'under, below') and 火 (ひ, 'fire'), with rendaku voicing of ひ to び. Literally 'low fire', it originally referred to a fire that has burned down and later extended to figurative decline.