Dirk is a Scots word for a long dagger; sometimes a cut-down sword blade mounted on a dagger hilt, rather than a knife blade. The word dirk could have possibly derived from the Gaelic word sgian dearg (red knife). It may also have been a corruption of the Low German words Dulk or Dolk. The shift from dearg to dirk is very minimal.
In Bronze Age and Iron Age Scotland and Ireland, the dirk was actually considered to be a sword. Its blade length and style varied, but it is generally 8-14 inches. However the Irish version often went as long as 21 inches in blade length...