Average adult A swings a sword as hard as they possibly can, Average adult B raises their hand to "catch" the sword and the sword comes into contact with Average adult B's hand. Does the sword slice through the hand? cut to the bone? Does the type of sword make a difference?
What sword are you thinking? With my non-existent knowing of this kind of stuff, I would say that the sword would go at least to the bone. Maybe it matters where on the hand the sword goes through as well?
Short answer, Adult B's hand is flying right off or at worst hanging on by a piece of flesh. Long answer, most swords are designed to cut through flesh and bone, with many having large or wickedly sharp cutting edges just for this purpose. The Greek sword, a kris, is the most deadly single-edged weapon in the world due to its large cutting edge and the blade being angled in a way for optimal chopping action. Most straight-edged blades are designed for hurting people, sure, but they also have uses like disarming opponents or piercing the chinks in armor plating.
Swords can do a lot depending on how they're made, but given the information you provided, Adult B is totally losing a hand.
Good to know, Thanks for the info. :)