The broad muscle on the side of the back, nicknamed the 'lats', has what full Latin name?

The broad back muscle nicknamed the “lats” is formally called the latissimus dorsi.

The name comes from Latin: latissimus means “broadest,” while dorsi refers to the back. A pair of latissimus dorsi muscles covers much of the middle and lower back, extending toward the sides beneath the arms. Their broad, flat shape creates much of the V-shaped torso associated with strength training.

The lats attach broadly along the spine, lower ribs, thoracolumbar fascia, and part of the pelvis, then narrow into a tendon that inserts on the humerus, the upper-arm bone. They help pull the arm downward and backward, draw it toward the body, and rotate it inward.

Pull-ups, chin-ups, lat pulldowns, and rows all train the latissimus dorsi, although they also involve muscles such as the biceps, teres major, trapezius, and rear deltoids. A common mix-up is calling every upper-back muscle a lat: the trapezius and rhomboids are separate muscles with different locations and functions.