An unimportant action that is the start of a major development, often an undesirable one. This expression alludes to the pointed metal wedge inserted into a log for splitting wood. The expression began to be used figuratively in the nineteenth century. Anthony Trollope used it several times in Dr. Thorne (1858), and it was commonplace by the end of the century. Richard Blackmore had it in The Remarkable History of Sir Thomas Upmore (1884): “My father kept calling him . . . the thin edge of the wedge, and telling dear mother . . . not to let him in.”