From the WSJ:
1. Place whole chicken on a work surface, breast-side down, thigh end closest to you. Starting at thigh end, cut along one side of backbone with kitchen shears or strong scissors. Repeat along other side. Discard backbone or save it for stock.Spatchcock is an 18th century term:
2. Turn the chicken over, breast-side up, and splay it open it like a book on a work surface. Use your hand to press hard on the chicken, cracking the breast bone, to flatten.
3. And that is a spatchcocked chicken, butterflied and flattened for even cooking in the oven, on the grill or under the broiler.
Alan Davidson explains in The Oxford Companion to Food: "The theory is that the word is an abbreviation of 'dispatch the cock,' a phrase used to indicate a summary way of grilling a bird after splitting it open down the back and spreading the two halves out flat." Davidson speculates that spatchcocked birds originated in Ireland. He has noticed them in Irish cookbooks that date to the 18th century.Everything old is new again.