fire edge
/ˈfʌɪə(r) ɛdʒ/noun
1The cutting edge of a newly-forged blade hardened by fire. Chiefly figurative : keenness, freshness, or vitality; the quality of being hard, stern, or inflexible. Now regional.
2The outer extent of a wildfire, especially the leading edge.
Origin
Late 16th century; earliest use found in John Knox (c1514–1572), religious reformer. From fire + edge.