After Michelle's comment about toeing (not towing) the line, I thought I'd add something about a tinker's dam. I'd read something about how a tinker used a bit of clay to help him "flood with solder" the metal he was repairing. The clay would be useless afterwards and so you have a useless dam.
I remember laughing once when someone wrote about a "tinker's darn".
According to the folks at this website, however, tinkers were known for their cursing. (My father always talked about cursing like a one-eyed carpenter. He had to explain the whole one eye, no depth perception, mashed fingers part to me.) Long before there was talk of a clay dam, there were expressions like "tinker's curse". Apparently, tinkers had a wide and varied vocabulary after all.
Oh well. I liked the "dam" story better- it gave you something to talk about.