h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The knife is called a sghian dubh in Gaelic (pronounced skiin' do (approximately)) which means "black knife" and apparently tucking it in your sock is a sign of friendship, which may say something about the state of cameraderie north of the border.

Back in the days when the law helped those who helped themselves, it was customary for a man to go about armed. While it was obviously somwhat less than amicable to enter someone's house armed to the teeth, a visitor would have to be unnaturally trusting to do so in a state of complete helplessness. The compromise was therefore that he would leave his major weaponry at the door, keeping only his sghian dubh. This, normally secreted in one's clothing somewhere in the vicinity of one's armpit, would be brought out and tucked in the sock where everyone could see it and reassure themselves that it was not about to be stuck into someone it shouldn't be....