muscles, meat
宍 centers on the physical substance of flesh: muscles and meat. It refers to the body's soft tissue, whether as living muscle or as food.
宍 is a variant form of 肉 (meat, flesh). The modern shape likely developed as a simplification or cursive-influenced variant, with 宀 and 六 replacing the original pictographic form.
Under a roof 宀, six 六 cuts of meat are hanging to dry. Picture a butcher's shed with six pieces of muscle meat.
For ニク, imagine a butcher poking the meat and saying 'neat cut': neat cut -> ニク, and the six pieces under the roof are neatly cut muscles.