As a boy I was always called Duncan, my first name. I was told that in Gaelic it means "brown warrior." The surname, Pirie, means rock or stone in Gaelic, and is thus one of the most common boys' forenames, as a variant of Peter, Petros, Pierre, Pedro, Poytr, Patrose, and others. I was Duncan growing up and at school.
At university I fell in with a group of friends who took pleasure in giving us all in-group names other than the ones we were usually known by. It was a kind of tribal thing. In Frank Herbert's book, "Dune," Paul Atreides chooses a tribal name "Paul Muad'Dib" to be known by amongst the Fremen. In my case it was the middle name Madsen. Like several of the others, the tribal name stuck. It was the surname of the mother who died when I was 2, and of whom I have no memories. It was also the married name of the Grandmother who raised me. And of course it was the name of my grandfather, Nils Madsen, the illegitimate Danish son of a vicar's daughter, who ran away to sea at age 15 and went on to become a merchant sea captain and a fellow of Trinity House. It is a name I bear with fondness for its history. It's how I came to have a surname as my Christian name and a Christian name as my surname.