Dirk is a Scots word for a short 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), "dearg" [ˈdƷʲɛrəg] shifting to "dirk" [dʌrk]. It may also have been a corruption of the Low German terms Dulk or Dolk.
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 was generally 7-14 inches (200 to 300 mm). Scottish dirks can range from less than 6 inches (150 mm) to around 20-25 inches (600 mm). However, the blades of Irish versions often were as much as 21 inches (530 mm) in length.
''... and if I do so may I be cursed in my undertakings, family and property, may I be killed in battle as a coward, and lie without burial in a strange land, far from the graves of my forefathers and kindred; may all this come across me if I break my oath."
Nearly every Scottish male at the time of the oath had a dirk. The dirk was small and was carried everywhere the owner went. The dirk was worn in plain view suspended from a belt at the waist.
Another shorter dagger tucked into a coat sleeve or stocking as part of Highland dress is known as a Sgian Dubh, derived from the arm pit dagger or sgian achlias. To this day, a real or false dirk is sometimes worn as a part of traditional Scottish costume.
In addition to these traditional weapons, other styles of dirk were worn by European and American forces on land and sea. Easier to carry than swords, dirks gained favor as lighter side arms among many military and naval officers during the 17th through 19th centuries. In some navies, they continued to be worn by midshipmen and cadets well into the 20th century. Numerous examples of naval dirks have survived from the earlier age of sail, some with histories of use during naval engagements. Most naval dirks were worn primarily on dress occasions, however, and consequently although attractive many were not designed for use in battle.
Naval and other dirks were commonly made with either double-edged or single-edged blades, and there was no standard blade configuration. Reference books covering naval dirks invariably show the popularity of both blade types. As a consequence, historically there were about as many naval dirks mounting single-edged blades as those with double-edged blades. Some dirks have single-edged blades that also have a false edge near the tip, a feature that could be useful in a backcut.
In the United States, dirk is a term used in the knife laws of several states. While it generally means any double-edged knife, the legal meaning may vary from state to state.