Not worth a tinker's dam

A tinker's dam, first recorded in 1877, was a wall of dough raised around a spot where a metal pipe is being repaired so as to hold solder in place until it hardens, whereupon the dam is discarded. However, tinker's damn was first recorded 1839 and probably was merely an intensification of "not worth a damn", rather than having anything to do with the dam (Christine Ammer: Dictionary of idioms)