A type of prosthetic used in the Fullmetal Alchemist world, which is the best manga/anime to have ever existed. Could be just a replacement arm, or even a shotgun/machinegun/missile launcher for your leg. Maybe just a new finger? Who knows? It can replace any thing.
"Oh no! My d@#$ got bitten off by a dog!"
"Don't worry, get an auto-mail d@#$ to repace it!"