Dead as a doornail

This very old expression has been around since the mid-14th century where it first appears in Pier’s Plowman (1350) and refers to the heavy studded nails on the outside of doors. Whether these nails are dead because the doorknocker has struck them on the head repeatedly or whether the heads of the nails have been turned or clinched thus making them dead in the sense that they cannot be extracted and re-used, is still hotly debated by etymologists.