flanc

Old French flanc, possibly from Frankish or Germanic origins (related to Old High German hlanca, hip), meant the side of the body from the hip to the lowest rib. The word was anatomical before it was military: the flanc was the soft side, the part of the body not covered by ribs or hip bone. Horses had flancs; men had flancs; armies discovered that so did formations.

Military Latin adopted the concept as flancus, and English military writers by the 14th century used flank for the side of a military formation — the vulnerable edge where an army was exposed to attack. Flanking an enemy meant attacking their side rather than their front: striking where they were thinner and less prepared. The tactical principle was as old as warfare itself, but the word gave it a name.

To outflank an enemy became the essential goal of battlefield maneuver — the enveloping movement that Alexander, Hannibal, and Napoleon all used to devastating effect. Hannibal's double envelopment at Cannae in 216 BCE, where he allowed the Roman center to push forward while his wings wrapped around both sides, destroyed a Roman army of perhaps 50,000 men and became the textbook case of flanking.

Flank has moved beyond the battlefield into everyday speech. To flank someone in a meeting, to flank a position in a debate, to be flanked by colleagues in a photograph: the side-attack and the side-presence both survive in the word. The body's soft side and the army's exposed edge remain in every modern use.