means A large, fast-spreading fire that burns through forests, grasslands, or other wild vegetation, often uncontrolled.

from A plain compound of "wild" and "fire," both Old English at heart — "wild" (wilde) for the untamed and "fyr" for flame. The pairing is old, and for centuries "wildfire" also named a sticky, near-unquenchable incendiary mix used in war (a cousin in spirit to Greek fire). From that ferocity comes the phrase "to spread like wildfire," carrying the sense of something that races outward faster than anyone can stop it.