Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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