Where did the sayng tinkers dam come from?

When someone refers to something as a tinker's dam, they are saying that it is worthless. The phrase came from men who used to travel from town to town making repairs on pots and pans. The molds these men used were called dams, and they were thrown away after just one use. It was the fact that these dams were so readily disposed that the term tinker's dam came to be.