A speedbag is usually filled with air or just rubber and mounted on the ceiling or wall, that way you can punch it quickly and get a rhythm going. It helps with coordination and works off fat on the arms.

[She can't help but blink at that.] You're a boxer and you don't know what a speedbagis? But really, you're a boxer?

[Genuine surprise is genuine. This girl looked nothing of the sort, maybe a little on the prissy side, but mostly cutesy.]