noun
A chicken or other fowl that has been split open and flattened for easier grilling or cooking.
verb
1. To prepare a dressed fowl for roasting or grilling by splitting open.
2. (British) To insert, interpolate, or sandwich in, especially in a forced or incongruous manner.
The process is similar to butterflying, but 'spatchcock' is only used in reference to fowl. The way I've seen it described, the spine is removed, and then the breastplate either split or removed. The resulting flattened bird cooks faster - which give the white meat less time to dry out.
Etymology: Unclear. Some imply it's a shortened version of "dispatch cock". Some imply it's derived from 'spitchcock', a split grilled or fried eel. There appears to be no known origin of 'spitchcock'.
There are some references - including Wikipedia and my old World Book Encyclepdic Dictionary - to 'spatchcock' originally referring to a culled immature chicken.

The word showed up in this week's newsletter from the fancy butcher I sometimes go to: I can order a holiday turkey and they will spatchcock it for a fee.