..I didn't know that tinkers built dams.

oh, I was hoping somewo'n would comment on that!
the word detective: [A] second theory maintains that the phrase should be "tinker's dam," not "damn." A tinker's "dam," goes this theory, was a small piece of dough, clay or paper used to block the hole in a pot while solder was applied. When the job was done, the "dam" was discarded, and thus "tinker's dam" came to mean something utterly inconsequential.



(gee, I wo'nder if this is a yart...

more than that; a self-yart!)
http://wordsmith.org/board/showthreaded.pl?Cat=&Board=words&Number=2432