psoas(n = noun.body) - either of two muscles of the abdomen and pelvis that flex the trunk and rotate the thigh;
is a kind of skeletal muscle, striated muscle

   An internal muscle arising from the lumbar vertebræ and inserted into the femur. In man there are usually two on each side, and the larger one, or great psoas, forms a part of the iliopsoas. [1913 Webster]

psoas, n. either of two muscles used in flexing the hip joint.

Etymology:
Gk, accus. pl. of psoa, taken as sing.