Exactly what the damn title says! This means to go Spanish Inquisition on someone's ass: thumbscrews, hooks, hot irons, needles, and anything else that reminds you of Saw or Hostel. Just picture Hellraiser, and you're Mr.Pinhead. The person you go medieval on is your own poor Frank Cotton.
"What now? Let me tell you what now. I'ma call a coupla hard, pipe-hittin' niggers, who'll go to work on the homes here with a pair of pliers and a blow torch. You hear me talkin', hillbilly boy? I ain't through with you by a damn sight. I'ma get medieval on your ass."