in cookery, denoting a method of grilling a game bird after splitting it open along the spine and laying it flat; a word of obscure origin.

It originated in Ireland in the late eighteenth century as a noun, referring to the bird thus dispatched, and indeed it may have been based on the verb dispatch, with the addition of cock. Another probable influence is the earlier spitchcock, a word of mysterious origin denoting similar treatment meted out to eels and other fish. [Ayto, "Diner's Dictionary"]
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