psoas n.

muscle in the loins or pelvis, 1680s, from Greek psoa (plural psoai) "muscles of the loins." Related: Psoitis; psoitatic.

Gk. [psoas], the gen. of the feminine noun [psoa], was mistaken by the French anatomist Jean Riolan (1577-1657) for the nom. of a (nonexistent) masculine noun. It was he who introduced this erroneous form into anatomy. [Klein]