Idiom Definitions for 'Not worth a tinker's dam'
This means that something is worthless and dates back to when someone would travel around the countryside repairing things such as a kitchen pot with a hole in it. He was called a 'tinker'. His dam was used to stop the flow of soldering material being used to close the hole. Of course his 'trade' is passÚ, thus his dam is worth nothing.
Idioms similar to 'Not worth a tinker's dam'