Something the hard lads at school devoted much time and effort to achieve. By vigorously rubbing the skin on the back of your hand with a two pence piece (tails down was best) you could friction-burn away the top few layers of skin. When repeated enough times this would lead to a much-admired thick brown scab about a cm wide and up to an inch long. One of the more unhinged hard knocks at my school had perfected this art to such a degree that both his lower forearms came to resemble Tony the Tiger's hind legs ... At the time it made no sense either.

See also: Biro burns -furiously scribble a biro in the back of a text book or on a bit of cardboard for a few minutes then press the hot nib into the skin on the back of your hand to give yourself an everlasting freckle.

Some kids in my school gave themselves haggard looking smiley faces. I didn't. I wasn't that stupid, even then.

The hardest kid that I know managed to give himself a burn in the shape of a J. His name was Simon. Note that Simon does not begin with, or contain, a J.
Five years later, though, that stubborn little J is still there. Simon shows neither regret nor pride. Or any evidence of really having thought about it.