Re: In my understanding, it is a tinker's dam, not damn. Meaning a small piece of material placed to dam the flow of liquid (melted) solder, confining the liquied to place desired. For this purpose one would use a disposable, insignificant bit of material, a useless bit of no other worth; hence, "not worth a tinker's dam".

a tinker, of course being the person who mended the pots and pails, usually an ititerant worker. there is a bawdy ballard about they hansome young tinker who hammers away in m'lady's chamber.. setting thing right..

but the word tinker does mean to take apart, and put back together and make minor repairs.. a jack of all trades..