A Skean is a short but rather vicious dagger generally worn in a ceremonial fashion.
The most famous of which is a Skean-Dhu coming directly from the Gaelic Sgian meaning dagger and Dubh meaning black which could allude to it's dark or hidden nature as traditionally the Skean-Dhu is secreted inside the sock.
notes that your Skean Dhu is always worn in the sock opposite your dominant
Nancy A. MacCorkill, (F.S.A. Scot.) believes that the Skean-Dhu may have given inspiration to the term blackmail as it was often the weapon of choice for covert and shady dealings.
However many believe that it may be simply derived from the coloration of the handle.
In more recent times the Skean-Dhu has been seen in less favourable light with calls for a less offensive imitation dagger to be used.
Most Scots find the very notion of this unacceptable, as the Skean-Dhu like many artifacts has more association with the culture and roots of Scottish culture than it has with being a secret weapon.