subscapularis

noun

sub·​scap·​u·​lar·​is ˌsəb-ˌskap-yə-ˈlar-əs 

: a large triangular muscle that fills up the subscapular fossa, that arises from the surface of the scapula, that is inserted into the lesser tubercle of the humerus, and that stabilizes the shoulder joint as part of the rotator cuff and rotates the humerus medially when the arm is held by the side of the body