You Can't Name This Muscle — Serratus Anterior

Serratus anterior — it sits on your ribs, but its job is on your shoulder blade. Your blade isn't bolted to anything: it floats on your rib cage, and this is the muscle that pins it there.

In this short: standing dumbbell shoulder press, converging machine shoulder press.

Also works your serratus: overhead press (dumbbell, barbell, machine), push-ups — especially pressing tall at the top, straight-arm cable pulldown, cable and dumbbell flies through the end of the squeeze, pullovers, ab-wheel rollouts and planks, punching and med-ball throws.