Translation:A sore arm
Yes, both of them, according to GPC.
braich from bracchium "arm", tost from tostus "roasted, toasted" via "burning" to "painful, severe, ill".
Cool, I didn't know tost was a possible Latin borrowing too!
I was once told that Welsh didn't distinguish between "arm" and "hand" and used a form of llaw for both. Sounds odd to us in this day and age but if you think about it, if you were a Brythonic warrior and you lost your fighting hand, you may as well have lost your whole arm.