1. The spanish dollar, manufactured in Mexico and Bolivia, made from silver from the spanish empire. The 'eight' refers to the number of pieces into which each coin could be cut in order to give change. The pirate connotation comes from the plundering of said pieces on the spanish main.
2. A songfight entry by rilla dope, the foremost pirate rap artists
We're headed for the silver train mate, where we be findin a load of pieces of eight.