To pistol-whip someone means to hit a person with the butt or barrel of a handgun (pistol), typically in the head or shoulder area. This is usually done in order to knock them unconscious or as a method of torture, sometimes to elicit information and sometimes to punish. A more archaic name for this maneuver is to buffalo.

Popular culture
Pistol-whipping is a well-known cliché in American popular culture, as it appears to be a simple method of asserting dominance. One of the best examples where pistol-whipping is seen as a method of asserting dominance is in the Martin Scorsese film Goodfellas in which Henry Hill (played by Ray Liotta), outnumbered three-to-one, brutally and mercilessly pistol-whips an adversary and as a result, the other two take no action. Harrison Ford's character Richard Kimble viciously pistol-whipped the villainous "one-armed man" in the critically acclaimed 1993 film The Fugitive. It was prominently featured in the film Equilibrium as a component of the fictional martial art of Gun Kata. James Bond also seems to be especially prone to pistol-whipping (he is rendered unconscious at least once, often by this method, in very nearly every film). In Star Trek franchise, actor Leonard Nimoy balked at his Mr. Spock using that cliché and proposed the Vulcan nerve pinch as an alternative.
On The Simpsons, when Snake threatens to pistol-whip Homer, Homer replies, "Mmm, Pistol Whip" and begins to imagine himself eating Cool Whip out of an oversized tub, using the pistol as somewhat of a spoon.
Contrary to fictional portrayals, there is no clear boundary between the force needed to render someone unconscious and the force needed to kill them, and all such blows to the head must be regarded as potentially fatal. In reality, anyone so frequently knocked out as James Bond, even if he were to survive, would inevitably suffer brain damage akin to the worst seen in the sport of boxing.