To pistol-whip someone is to hit them with a pistol, using the gun as a hard stick, not firing it. News stories, crime novels, and films use this a lot. It is a violent word; you will almost never hear it in polite everyday talk. "Whip" here does not mean a leather whip. It is the old idea of striking, often more than once. People picture blows with the handle (the heavy back end of the gun), though a blow with any part of the pistol can count. You can pistol-whip someone with a single hit, but the usual picture is several. If the gun goes off, that is shooting, not pistol-whipping.