Reach Messenger Bag from Travelon adds several layers of extra security on your messenger bag. One of it includes medieval technology: Medieval chainmail.
The bag doesn't look like a suit or armor, but it has slash proof outer layer that ensures no one with a sharp knife will be able to just hack open your bag and empty its contents. It's also got a cut proof shoulder strap making it harder to snatch while you're wearing it.
The React bag also uses locking caribiners to keep the main compartment closed, and to let you lock it up to a chair or table leg for extra security when it's off your person. Going for $100.