I hope this post doesn't run afoul of the rules, but I have a question. What exactly is a mattucashlass?

In my reading about sgian dubhs, I have come across references to a mattucashlass, or "armpit dagger", which was apparently sometimes carried in Scotland in days gone by. But what exactly is it and how was it carried? If it was just held under the arm, it would seem to me that it would be easily and quickly dropped, so there must be more to the story than that. Did those who carried them also carry a sgian dubh, or was it one or the other, but not both??