the.com/brush fire

means A fast-moving, low-intensity fire that burns through brush, shrubs, and dry undergrowth rather than tall forest — and by extension, any small trouble that flares up suddenly but is (usually) contained before it spreads into something bigger.

from Straightforward compound: 'brush' (the Middle English word for underwood, twigs, and scrubby vegetation — possibly related to Old French 'broce/broche') plus 'fire.' The literal sense — a fire that feeds on brush rather than full timber — is old and purely descriptive. The figurative use, for a minor conflict or crisis that ignites quickly and spreads fast but stays small (a 'brushfire war,' a 'brushfire rebellion'), caught on in 20th-century journalism and military language, borrowing the image of something that flares hot and fast across dry ground but, with luck, burns itself out before it becomes a forest fire.