A sgian-dubh is a small knife that people in Scotland wear with their traditional dress called a kilt. It's a special knife that's about the size of a grown-up's hand and it's usually made out of metal, like steel or silver. Sgian-dubh means "black knife" in Scottish Gaelic.
The knife is worn in a special pocket called a "sporran" that's attached to the kilt. The pocket is usually at the front or side of the kilt. The knife is pulled out and shown to people as a sign of respect and honor.
The sgian-dubh has a sharp pointy blade which is used for cutting things like food, but it's also a symbol of the Scottish culture and history. Sometimes, the knife has a fancy design on it, or a special handle made out of wood, bone or even animal antlers.
People who wear the sgian-dubh take good care of it, sharpen it often so that it stays sharp and shiny. They never use it for anything bad, like hurting other people, because it's a symbol of their heritage and they want to treat it with respect.