A hatpin is a long, thin pin used to keep a woman’s hat on her head. One end is sharp; the other often has a pretty head (glass, metal, or a small jewel). You push it through the hat and into the hair so wind cannot lift the hat. This was everyday kit when women wore large hats; you still see them in museums, period films, and with some formal hats. People used to joke that a hatpin could also be a weapon in a crowded street — that is part of the old city folklore around it. It is a real object-name, a little old-fashioned, not slang.