flank (flăngk) n. 1. The section of flesh on the body of a person or an animal between the last rib and the hip; the side. 2. A cut of meat from the flank of an animal. 3. A lateral part or side: the flank of a mountain. 4. a. The right or left side of a military formation: an attack on both flanks. b. The right or left side of a bastion. tr.v. flanked, flank·ing, flanks 1. To protect or guard the flank of.