Rashi on Leviticus 7:32/רש"י על ויקרא ז׳:ל״ב

1שוק is that limb of the right rear-leg extending from the knee-joint, the bone and blesh of which are usually sold as offal together with the head, unto the middle joint which is what is known as the סובך of the leg (the סובד is the small bone between that middle joint and the thigh-bone; consequently the שוק spoken of in connection with שלמים is the middle one of the three limbs that form an animal’s leg) (Chullin 134b).