Emily Dickinson Lexicon

FLANK, n. [Fr. flanc; Sp. and Port. flanco; It. fianco; G. flanke; Sw. and Dan. flank; Gr. λαγων; probably connected with lank, W. llac, Eng. flag, Gr. λαγαρος, and so called from its laxity, or from breadth.]

1. The fleshy or muscular part of the side of an animal, between the ribs and the hip. Hence,
2. The side of an army, or of any division of an army, as of a brigade, regiment, or battalion. To attack an enemy in flank, is to attack them on the side.
3. In fortification, that part of a bastion which reaches from the curtain to the face, and defends the opposite face, the flank and the curtain; or it is a line drawn from the extremity of the face toward the inside of the work. Harris. Encyc.