A flank is the side of something, whether of beef or a military formation.
Although flank is specifically used to talk about a cut of meat, or the right or left side of a military formation, we also use it in those same senses for other objects. If you've put on weight, it's probably not just your tummy sticking out a bit more, but the lovehandles on your flanks. If you see someone walking down the street with bodyguards on either side, she's flanked by guards.
