A pin is a small, sharp device that's made for attaching and holding things together. When you sew a shirt, you first have to pin the paper pattern to the fabric with pins.
Pins used for tacking paper to a bulletin board are made of sharp metal. A piece of jewelry that fastens to your shirt is also a pin. There are numerous other pins, from the pegs a guitar's strings are attached to; to the wooden pins you try to knock down when you're bowling; to the pins that hold your hairdo in place. And when you pin, you secure with a pin, or you hold someone down while wrestling.