What exactly is a mattucashlass?

[Orvis]

I agree with Luke as far as the hideout knife was concerned. I, too, have a replica of this knife as illustrated in Wallace's "Scottish Swords and Dirks", p.77 (Luke, is your sgian one of Glenn McClain's blades? It looks very much like mine). With the scabbard worn hooked over the arm opening of a waistcoat (or secured within the sleeve of a jacket in some other manner), it would make a most effective concealed secondary weapon in case the Highlander were disarmed of openly-carried weapons. When I studied Gaelic, I never ran across the term "mattucashlass" for this weapon. I've known it as a "sgian-achlais" (armpit knife, from Dwelly's Gaelic-English Dictionary, p. 822). I also agree with Luke that the sgian dubh did not appear as an item of Highland dress (for those who could afford it) until the very end of the 18th century-beginning of the 19th century, and probably evolved from the gralloching knives carried by huntsmen. After all, who needed a little sgian dhu when one had a biodag (dirk) and maybe a sgian-achlais?