the.com/grassfire

means A fire that burns quickly through grass or brush, spreading fast along the ground rather than burning slow and deep.

from A straightforward compound of 'grass' plus 'fire' — English loves to just smash two nouns together and let context do the work. The word's vividness comes less from hidden roots than from the real phenomenon it names: dry grass catches and spreads with alarming speed, which is why 'spread like a grassfire' became a ready-made metaphor for anything — rumor, panic, fashion — that races through a population before anyone can stop it.