Matambre

Matambre is the name of a very thin cut of beef in Argentina, Uruguay, Rio Grande do Sul and also Paraguay. It is a rose-colored muscle taken between the skin and the ribs of the steer, a sort of flank steak. It is not the cut known normally in the U.S. as flank steak. Some people refer to it as the "fly shaker", because it is the muscle used by the animal to twitch to repel flies and other flying... (Wikipedia)