The expression is pretty ancient and means to be as if dead. Its origins date back to 1350, and it has also appeared in some of the 14th century work, like The Vision of Piers Plowman as well as in Henry IV by Shakespeare. According to the The Morris Dictionary of Word and Phrase Origins, the term is said to have been derived from the action of hammering a nail through wood and then flattening its end over the inside so it cannot be removed or fidgeted with again, which eventually leaves the nail dead, as its useless once clinched.