Probably a concatenation of "dispatch" and "cock," it means simply that you've removed the bird's backbone in order to flatten it out, so that now the aforementioned geometry has been converted from a pain-in-the-ass protuberant ovoid to something closer to flat. The reduced overall thickness means shortened overall cooking time, and thus a smaller disparity (i.e. margin for error) between cooking times for dark and light sections. Moreover, whereas a whole chicken necessitates ambient heating (roast in an oven, for example), the flattened shape allows for direct-heat cooking methods, which is ideal for creating a nice, browned crust. Back in the day, you might imagine some very hungry folks not wanting to bother with actually butchering the bird, cutting and butterflying it out, skewering it with a couple of sticks, and cooking it right over an open fire. You could also imagine these folks picking a smaller bird so that it might cook even faster, and indeed in some parts of the world "spatchcock" is a noun referring to a smaller-sized chicken. The slightly more pervy among us might have noticed that a spatchcocked chicken, all splayed out as it is, could strike one as somewhat . . . suggestive. And indeed, I came across at least one instance in literature where someone gets "banged . . . spatchcock," which I can only assume means spread-eagled or something. Sheesh, and I was so trying to stay clear of this sort of cheap ribaldry. But seriously, heh heh. "Spatchcock."