A "heavy bag" is a durable, weighted punching bag used in boxing and martial arts training. It builds power, timing, and endurance by absorbing strikes. It hangs from a stand or ceiling and is packed with sand, foam, or fabric.
There used to be an old thing where every team had a heavy bag in their locker room for people to punch, but again, it was more about conditioning because if you hit a heavy bag for a minute, it feels like your arms are about to fall off. Steve Yzerman
Life is a long journey with a heavy bag on its back. Japanese Proverb