flank
n. countablen. the side of something, like a person's body, a large animal, or a group of soldiers. It is the area between the front and the back.
n. the side of a person's or animal's body between the ribs and the hip; by extension, the right or left side of a military formation or a large structure.
The horse has a white patch on its left flank.
The general ordered the cavalry to attack the enemy's weakened left flank to break their line.
The hikers struggled to navigate the steep western flank of the mountain as the sun began to set behind the jagged peaks.
From Late Middle English flanc, from Late Old English flanc (“flank”), from Old French flanc, of Germanic origin, probably Frankish hlanca, from Proto-Germanic hlankō (“bend, curve, hip, flank”), from Proto-Germanic hlankaz (“flexible, sleek, bendsome”), from Proto-Indo-European kleng- (“to bend”). Akin to Old High German hlanca (“loin”), Middle Low German lanke (“hip joint”) (German lenken (“to bend, turn, lead”)), Old English hlanc (“loose, slender, flaccid, lank”). More at lank.
Often used in military or anatomical contexts to describe the side of a formation or body.