(1) A fire found in lightweight perennial vegetation such as bushes, shrubs, and scrub growth. A brush fire may not be as large or intense as a forest fire, but you can't really bet your life on that.
(2) Also, fire in a brush recently used by a backpacker fresh off the trail.
This brush may have been used on the head, but more frequently on another part of the pelt (backpackers are known for growing excessive amounts of unduly long body hair).
Ignition is often spontaneous, triggered by oxidation of natural body oils which can't be properly removed by simple bathing, or by bathing and diligent scrubbing. Or even by a trip through an industrial clothes washer, though some try this, and it can be a source of temporary amusement during a long, dull, rainy weekend in some random town.
Rooms in lodgings along thru-hiking routes normally come supplied with sturdy airtight metal cans for disposal of body hair and used brushes, but carelessness often results in tragedy when a hair-clogged brush bursts into flames deep in the night (anytime after 8 p.m.).
As always, Effort or Eff it. No sniveling.
Source: How to talk in the woods.