Back muscle, informally Crossword Clue

 The answer to the crossword clue "back muscle, informally" is lats.

The latissimus dorsi (lats) is a large, triangular muscle that covers the back. It is responsible for pulling the arms down and back, as well as helping to rotate the torso. The lats are one of the most important muscles for pulling movements, such as rows and pull-ups.

The word "lats" is a shortening of the term "latissimus dorsi". It is often used informally to refer to the back muscles in general.

Here are some other informal terms for back muscles:

Wings

Wingspan

Back wings

Lats

Latissimus

I hope this helps!