The Collaborative International Dictionary of English v.0.48:

Psoas \Pso"as\, n. [Gr. ? a muscle of the loin: cf. f. psoas.]
   (Anat.)
   An internal muscle arising from the lumbar vertebr[ae] 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]

WordNet (r) 3.0 (2006):

psoas
    n 1: either of two muscles of the abdomen and pelvis that flex
         the trunk and rotate the thigh